PDMS-based ligands for quantum dots in silicones

ABSTRACT

The invention provides a process for the production of a wavelength converter containing a siloxane polymer matrix with wavelength converter nano particles embedded therein, the process containing (a) mixing (i) a first liquid containing (i1) short chain siloxane polymers and (i2) wavelength converter nano particles having an outer surface grafted with siloxane grafting ligands and (ii) curable siloxane polymers, and (b) curing the curable siloxane polymers, thereby producing the wavelength converter (100); wherein the short chain siloxane polymers have s1 Si backbone elements, wherein the siloxane grafting ligands comprise siloxane grafting ligands having x1 Si backbone elements, wherein at least one Si backbone element of each siloxane grafting ligand comprises a group having a grafting functionality; and wherein the curable siloxane polymers have y1 Si backbone elements, wherein x1/s1≥0.8, s1&lt;y1 and wherein x1&lt;y1.

CROSS-REFERENCE TO PRIOR APPLICATIONS

This application is the U.S. National Phase application under 35 U.S.C. § 371 of International Application No. PCT/IB2013/058785, filed on Sep. 24, 2013, which claims the benefit of U.S. Patent Application No. 61/718,260, filed on Oct. 25, 2012 and U.S. Patent Application No. 61/767,877 filed on Feb. 22, 2013. These applications are hereby incorporated by reference herein.

FIELD OF THE INVENTION

The invention relates to a process for the production of a polymer with within the polymer embedded wavelength converter nano particles, to a wavelength converter obtainable with such process, and to a lighting unit comprising such (polymeric) wavelength converter.

BACKGROUND OF THE INVENTION

The use of nano particles, such as quantum dots (QD), for lighting applications is known in the art. US20110240960, for instance, describes a light emitting device comprising a light emitting source, a first quantum dot wavelength converter disposed above the light emitting source, the first quantum dot wavelength converter comprising, a plurality of first quantum dots for generating wavelength-converted light by converting wavelength of light from the light emitting source, a first dispersive medium embedding the first quantum dots dispersively therein, and a first sealer for sealing the entire outside surface of dispersive medium embedding the first quantum dots in a pack.

A first encapsulant is applied for encapsulating the entire outside surface of the first quantum dot wavelength converter. Further, a second quantum dot wavelength converter is disposed above the first quantum dot wavelength converter, the second quantum dot wavelength converter comprising a plurality of second quantum dots for generating wavelength-converted light by converting wavelength of light from the light emitting source, a second dispersive medium embedding the second quantum dots dispersively therein, and a second sealer for sealing the entire outside surface of the second dispersive medium embedding the second quantum dots in a pack, wherein the first quantum dot wavelength converter, the second quantum dot wavelength converter and the light emitting source are spaced apart from each other. The second encapsulant is deposited on the entire outside surface of the second quantum dot wavelength converter and for encapsulating the entire outside surface of the second quantum dot wavelength converter. Further, the light emitting source is a light emitting diode or a laser diode.

SUMMARY OF THE INVENTION

Nano particles, such as quantum dots (QDs), have shown to be highly interesting in lighting applications. They could e.g. serve as inorganic phosphor(s) in converting blue light to other colors and have the advantage of a relative narrow emission band and the advantage of color tunable by the size of the QDs to be able to obtain high quality pure white light. In order to use QDs for LED applications, they need to be incorporated in a suitable matrix. A QD powder (without matrix) is in general not desired because of both concentration quenching effects and poor process ability of such a pure QD powder. Up to now, embedding of nano particles in many types of polymers appears to lead to aggregation of the nano particles. Currently, acrylic matrices are mostly used as a matrix for QDs, but they are known for their poor stability towards high blue light fluxes. We consider silicones the most preferred matrix for QDs because of the proven stability of silicones towards high blue fluxes (i.e. their proven compatibility with LEDs).

Silicones are currently used as standard matrix/resin for many LED-manufacturing processes. However, QDs generally have a hydrophobic organic coating (in the form of ligands, generally extending from the outer surface of the QD) which make them incompatible with silicones: generally a turbid mixture is obtained when QDs are mixed with silicones caused by agglomeration of the QDs. This is undesired because of concentration quenching effects, expected increased degradation effects, and an uncontrolled way of processing these films leading to spatial concentration variations. To the best of our knowledge, no example of QDs (with coordinating ligands) that are truly miscible with optical silicones exists.

In summary, a more general way of improving miscibility of QDs to optical silicones is highly desired. Hence, it is an aspect of the invention to provide an alternative nano particle—polymer system, especially a polymer—quantum dot system. Especially, it is an aspect of the invention to provide an alternative process for the production of such polymer with embedded nano particles. Further, it is an aspect of the invention to provide an alternative wavelength converter with nano particles embedded therein. Yet, it is a further aspect to provide an alternative lighting unit comprising such polymer with embedded QDs. Preferably, the alternative process and/or alternative wavelength converter and/or alternative lighting unit at least partly obviate one or more of above-described (and also further below described) drawbacks of prior art solutions.

Surprisingly, the inventors have found amongst others that by exchanging the native QD ligands of wavelength converter nano particles by particular PDMS-alike ligands (PDMS=poly dimethyl siloxane), QDs may become truly miscible with silicones and/or significantly improves miscibility with silicones when specific conditions are met. Further, there is advantageously no need to use significant amounts of additional solvents such as hexane and acetone, or other solvents to obtain a well miscible system.

It further appeared to be beneficial that siloxane based ligand exchanged quantum dots, such as e.g. PDMS based ligand exchanged quantum dots, are first dispersed in a low molecular weight PDMS, optionally followed by removal of solvent, and in a second step mixed with a commercial silicone or other higher molecular weight siloxane, especially PDMS, while maintaining the colloidal stability. The low molecular weight siloxane, especially PDMS, can bear crosslinkable groups such as vinyl or hydride groups to enable crosslinking of the low molecular weight siloxane, especially PDMS, in the silicone matrix. Using this method, quantum dots can be well dispersed in a silicone matrix, even if it contains molecular weights higher than that of the ligand on the particle surface. Hence, instead of dispersing grafted nano particles directly in a curable siloxane now the grafted nano particles are first pre dispersed in a short chain siloxane. The mixture of predispersed grafted nano particles can well be dispersed in the curable siloxane polymers, which have much longer chains (than the grafting ligands). Would the grafted nano particles directly be dispersed in the curable siloxane polymers, bad distribution of nano particles are found and/or bad lighting characteristics. A solution would be to use longer grafting ligands, but this complicates the ligand exchange reaction.

Hence, in a first aspect, the invention provides a process for the production of a wavelength converter comprising a siloxane polymer matrix with wavelength converter nano particles (herein also indicated as “nano particles”) embedded therein, the process comprising: (a) mixing (i) a first liquid comprising (i1) short chain siloxane polymers (“S polymers”) and (i2) wavelength converter nano particles having an outer surface grafted with siloxane grafting ligands (“X polymers”) and (ii) curable siloxane polymers (“Y polymers”), and (b) curing the curable siloxane polymers, thereby producing the wavelength converter; wherein the short chain siloxane polymers have s1 Si backbone elements, wherein the siloxane grafting ligands comprise siloxane grafting ligands having x1 Si backbone elements, wherein at least one Si backbone element of each siloxane grafting ligand comprises a group having a grafting functionality; and wherein the curable siloxane polymers have y1 Si backbone elements; wherein x1/s1≥0.8, such as x1/s1≥0.95, such as >1, like at least ≥1.2, such as at least >2, wherein s1<y1, such as s1/y1<0.25, and wherein in a specific embodiment x1<y1.

It appears that first dispersing the grafted nano particles in short chain (curable) siloxane polymers and then combining with long change curable siloxane polymers, wavelength converters may be obtained with good lighting properties and a good distribution of the nano particles.

The nano particles are wavelength converter nano particles, which may especially be configured to provide, upon excitation by UV and/or blue light, luminescence in at least part of the visible part of the spectrum. Hence, these particles are herein also indicated as wavelength converter nano particles, of which QDs (quantum dots) are a specific embodiment.

Such wavelength converter, obtainable by the herein described process, may show luminescence (when embedded in the matrix of cured siloxane polymers) with a high quantum yield and stability. Further, the wavelength converter may be relatively temperature and/or photo chemically stable and/or transparent. Further, with this process, nano particles may be dispersed in the polymer in a relative even way, without the substantial disadvantage of agglomeration. Hence, in a further aspect, the invention also provides a wavelength converter per se, especially obtainable by the process of the invention. Especially, the invention also provides the light converter (per se) comprising a (cured) siloxane polymer (matrix) with wavelength converter nano particles embedded therein, wherein: (a) the wavelength converter nano particles have an outer surface grafted with siloxane grafting ligands, and (b) the siloxane polymer matrix comprises siloxane polymers of a first type and siloxane polymers of a second type, wherein at least part of the second type of siloxane polymers are cross-linked; wherein the siloxane polymers of the first type comprise short chain siloxane polymers having s1 Si backbone elements, wherein the siloxane grafting ligands comprise siloxane grafting ligands having x1 Si backbone elements, wherein the siloxane polymers of the second type comprise siloxane polymers having y1 Si backbone elements; wherein x1/s1≥0.8, such as x1/s1≥0.95, such as >1, like at least >1.2, such as at least >2, wherein s1<y1, such as s1/y1<0.1, and wherein in a specific embodiment x1<y1.

As these wavelength converters may well be applied in lighting devices, the invention provides in yet a further aspect a lighting device comprising a light source configured to generate light source light (i.e. light from the light source), a wavelength converter as defined herein, especially obtainable by the process as defined herein, configured to convert at least part of the light source light into visible converter light. In yet a further aspect, the invention also provides a liquid crystal display device comprising one or more backlighting units, wherein the one or more backlighting units comprise one or more lighting devices as defined herein.

The term wavelength converter refers to a system that is configured to convert light from a first wavelength into light of a second wavelength. Especially, UV and/or blue light (excitation wavelength) may be (at least partially) converted into (visible) light of higher wavelength than the excitation wavelength. This will further be elucidated below; first some aspects concerning the siloxane polymer, siloxane grafting ligands and curable siloxane polymers are described, as well as embodiments of a process to obtain the wavelength converter.

Silicones, more precisely called polymerized or polymerizable siloxanes or polysiloxanes, are mixed inorganic-organic polymers with the chemical formula [(R₁,R₂)SiO]_(n) (not taking into account the terminal groups), where R is a group such as for example hydrogen, hydrocarbon or fluorocarbon, especially methyl, ethyl, or phenyl. Especially, one or more R groups of one or more Si backbone elements comprise one or more of hydrocarbon and fluorocarbon. One or more of these side groups may also have cross-linking functionality, such as a vinyl group or a hydride group. These polymerized siloxanes or polysiloxanes materials consist of an inorganic silicon-oxygen backbone ( . . . —Si—O—Si—O—Si—O— . . . ) with organic side groups attached to the silicon atoms, which are four-coordinate. As the R side groups may in principle be different, instead of the formula [(R₂)SiO]_(n) also the formula [(R₁,R₂)SiO]_(n) (not taking into account the terminal groups), might be applied. Note that herein x1 and y1 are applied for the number of Si elements in the siloxane backbone for the siloxane grafting ligands and (curable) siloxane polymers (that form the host material), respectively. Likewise, s1 is herein used for the number of Si elements in the siloxane backbone of the short chain siloxane(s) (S polymers). Especially, the backbone of a polymer is the series of covalently bonded atoms that together create the continuous chain of the molecule. For instance, [(R₂)SiO]_(n) has n Si backbone elements and n O backbone elements.

The fact that herein only R, or more precisely, R₁,R₂ are mentioned, does not exclude that different Si backbone elements may comprise the same side groups, but also more than two different types of side groups may be comprised by the silicone. Hence, R may for instance, but not limited to, be selected from the group consisting of methyl, phenyl, etc. Also halogens, mainly chlorine, are possible as side compound R. Further, [R₂SiO], or [—Si(R)₂—O—] refers to the silicone unit or silicone characterizing group (i.e. group that characterizes a silicone).

A siloxane is any chemical compound composed of units of the form R₂SiO, where R is for instance, but not limited to, a hydrogen atom, a hydrocarbon group, or one or more R₂SiO unit(s) combined with a terminal group. Siloxanes can have branched or unbranched backbones consisting of alternating silicon and oxygen atoms —Si—O—Si—O— with side chains R attached to the silicon atoms. Polymerized siloxanes with organic side chains (R≠H) are commonly known as silicones or as polysiloxanes. Herein, these are also indicated as “siloxanes” or “siloxane polymers”. Representative examples are [SiO(CH₃)₂]_(n) (polydimethylsiloxane) and [SiO(C₆H₅)₂]_(n) (polydiphenylsiloxane). These compounds can be viewed as a hybrid of both organic and inorganic compounds. The organic side chains confer hydrophobic properties while the —Si—O—Si—O— backbone is purely inorganic. As indicated above, Si elements in the backbone are herein also indicated as Si backbone elements. Hence, any siloxane characterizing moiety R₂SiO provides one silicon backbone element (which has two side groups). Note that e.g. PDMS is CH₃[Si(CH₃)₂O]_(n)Si(CH₃)₃, has n+1 Si elements, thus in fact n+1 Si backbone elements. Would such siloxane be used as grafting ligand, x1=n+1; would such siloxane be used as siloxane polymer for curing, y1=n+1. Further, PDMS (see formula) has n−1 non-terminal Si backbone elements. In case mixtures of siloxanes are applied, such as in the case of polydisperse siloxanes, s1, x1, and y1 may especially be averaged values, especially weight averaged values, respectively.

By varying the —Si—O— chain lengths, side groups, and cross linking, silicones can be synthesized with a wide variety of properties and compositions. They can vary in consistency from liquid to gel to rubber to hard plastic. The most common siloxane is linear polydimethylsiloxane (PDMS; see above), a silicone oil. The second largest group of silicone materials is based on silicone resins, which are formed by branched and cage-like oligosiloxanes. Herein, especially linear siloxanes are used as curable siloxane polymers and/or siloxane grafting ligands and/or short chain siloxane polymers. However, also non-linear siloxanes may be used as curable siloxane polymers and/or siloxane grafting ligands. Further, as the siloxanes are cured, in general the wavelength converter will be a solid wavelength converter (solid polymeric wavelength converter). Nevertheless, the wavelength converter might in an embodiment be flexible.

As indicated above, the siloxane grafting ligands comprise siloxane grafting ligands having x1 Si backbone elements; especially, the grafting ligands are siloxane grafting ligands (having x1 Si backbone elements). The term “grafting ligand” refer to a ligand that coordinates to or is bound to the outer surface of a wavelength converter nano particle (these particles are further elucidated below), such as quantum dots. Grafting ligands are e.g. known in the art, and are for instance described in WO/2009/035657, WO/2010/014198 and WO/2008/063653, etc. Grafting ligands are sometimes also indicated as capping ligands.

The grafting ligands comprise siloxane molecules, which will in general have the commonly known side groups, but also have at least one group, especially at least one side group, having a grafting functionality. The (side) group having a grafting functionality may be selected from the group consisting of an amine and a carboxylate. For instance, the amine can be —NH₂ or COOH, but may also be —R—NH₂ or R—COOH, respectively, wherein R is a hydrocarbon, preferably comprising less than 20 carbon atoms. However, the (side) group having grafting functionality may also comprise a phosphine, a phosphine oxide, a phosphate, a thiol, etc. (and in an embodiment combinations of two or more thereof). Hence, the grafting ligands are siloxane molecules, which will in general have the commonly known (side) groups, but also have at least one (side) group having a grafting functionality selected from the group consisting of an amine, a carboxylate, a phosphine, a phosphine oxide, a phosphate, a thiol, even more especially an amine, a carboxylate, a phosphine, a phosphine oxide, and a phosphate. The ligand may in an embodiment comprise a plurality of (side) groups having a grafting functionality, which may comprise different types of such (side) groups (or which may be all identical). A Si backbone element may also comprise two (or more) (side) groups having a grafting functionality. The phrase “side group having a grafting functionality” refers to a side group (not a terminal group) which has the ability to graft to a luminescent nano particle, as describe herein. Therefore, the (side) group having grafting functionality provides to the siloxane its grafting ability (and thus grafting ligand functionality). The one or more groups with grafting functionality are preferably side groups (see also below), though optionally one or more of such one or more groups with grafting functionality may also be end groups (or terminal groups). Hence, especially the side group is a (side) group of a non-terminal Si backbone element (see also below). Therefore, in other words, the grafting ligand especially comprises a non-terminal functional group at least having grafting functionality.

The amine may be grafted as amine to the outer surface of the luminescent nano particle; the carboxylate may be grafted as carboxylate to the luminescent nano particle. Especially, it appears that the functional groups should especially be arranged as side groups and not as end groups. Hence, especially, the grafting ligands comprise siloxane molecules that have end groups which do not comprise a group selected from the group consisting of an amine, a carboxylate, a phosphine, a phosphine oxide, a phosphate, and a thiol; i.e. not having end groups which (substantially) have grafting functionality. The siloxane grafting ligands have especially (side) groups with grafting functionality for the herein indicated semi conductor quantum dots, especially the herein indicated CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, HgS, HgSe, HgTe, CdSeS, CdSeTe, CdSTe, ZnSeS, ZnSeTe, ZnSTe, HgSeS, HgSeTe, HgSTe, CdZnS, CdZnSe, CdZnTe, CdHgS, CdHgSe, CdHgTe, HgZnS, HgZnSe, HgZnTe, CdZnSeS, CdZnSeTe, CdZnSTe, CdHgSeS, CdHgSeTe, CdHgSTe, HgZnSeS, HgZnSeTe, HgZnSTe, GaN, GaP, GaAs, AlN, AlP, AlAs, InN, InP, InAs, GaNP, GaNAs, GaPAs, AlNP, AlNAs, AlPAs, InNP, InNAs, InPAs, GaAlNP, GaAlNAs, GaAlPAs, GaInNP, GaInNAs, GaInPAs, InAlNP, InAlNAs, and InAlPAs nanoparticles, even more especially the sulfides, tellurides and selenides.

The (side) groups having a grafting functionality may be arranged anywhere over the siloxane backbone of the grafting ligand. Assuming a linear siloxane with x1 silicon backbone units, then especially the one (or more) (side) group(s) having grafting functionality are found between 20-80% of the backbone length. Assuming for instance the backbone comprising 50 Si backbone elements, especially the (side) group having grafting functionality is found on Si no. 10, or Si no. 40 or in between (with no's 1 and 50 being end groups).

As indicated above, there is at least one such (side) group, though optionally there may be more than one (side) groups having a grafting functionality such as selected from the group consisting of an amine and a carboxylate, or others, such as a phosphine, a phosphine oxide, a phosphate, a thiol. The number of such (side) groups having a grafting functionality may depend upon the chain length of the siloxane grafting ligand, but especially does not exceed the number of 10, especially not more than 4, such as 1-2. Hence, especially not more than up to 10 Si backbone elements (not being terminal Si backbone elements) of each siloxane grafting ligand comprise a (side) group having a grafting functionality. Especially not more than up to 10 Si backbone elements (not being terminal Si backbone elements) of each siloxane grafting ligand comprise a (side) group (having a grafting functionality) selected from the group consisting of an amine comprising side group, a carboxylate comprising (side) group, a phosphine comprising (side) group, a phosphine oxide comprising (side) group, a phosphate comprising (side) group, and a thiol comprising (side) group. When more than one side group having a grafting functionality is present, especially the percentage of (side) groups having a grafting functionality is equal to or less than 5 mole % (of all R1,R backbone side groups, not more than 5% comprises such functional group), even more especially the percentage of (side) groups having a grafting functionality is equal to or less than 2.5 mole %. Hence, assuming e.g. 22 Si backbone elements (thus including two terminal Si backbone elements), there are 40 (side) groups available; when 5% of them would have grafting functionality, this would imply that up to two (side) groups would have grafting functionality; the others would have no grafting functionality, such as methyl, phenyl, etc.

Note that the terms “grafting ligand” or “siloxane grafting ligand” may also refer to a plurality of different types of siloxane grafting ligands. In an embodiment, these siloxane grafting ligands are substantially identical. However, in another embodiment, the siloxane grafting ligands may comprise a plurality of different siloxane grafting ligands, such as a carboxylate and a phosphate. For instance, they may differ in chain length (x1), and/or they may differ in side groups, and/or they may differ in side groups having a grafting functionality, and/or may differ in the number of side groups having a grafting functionality and/or may differ in the position of side groups having a grafting functionality (and/or differ in the type of end groups). For instance, the siloxane grafting ligands may comprise a plurality of siloxane polymers, each having only one (amine) side group, but wherein the position of the (amine) side group is randomly distributed over the siloxane polymers.

In general, the curable siloxane polymers, or the (cross-linked) siloxane polymers of the wavelength converter (polymeric device) do not have one or more (side) groups having a grafting functionality selected from the group consisting of an amine and a carboxylate.

Except for the side groups having a grafting functionality, above (and below) indicated (general) information with respect to the siloxane grafting ligands substantially also applies to the curable siloxane polymers and/or short chain siloxane polymers.

The term “curable siloxane polymers” may also refer to a plurality of different types of curable siloxane polymers. In an embodiment, these curable siloxane polymers are substantially identical. However, in another embodiment, the curable siloxane polymers may comprise a plurality of different curable siloxane polymers. For instance, they may differ in chain length (y1), and/or they may differ in (the type of) side groups. Further, they may differ in the type of end group. The curable siloxane polymers may have (end) groups that are configured to form cross-links upon curing. Note that additionally or alternatively, also one or more side groups per curable siloxane polymer may be configured to form a cross-link upon curing. For instance, the side groups may include a vinyl group (or a hydrogen group). As can be understood from the above, the curable siloxane polymers may comprise end groups and/or side groups that are configured to form cross-links upon curing.

The term “short chain siloxane polymers” may also refer to a plurality of different types of short chain siloxane polymers. In an embodiment, these short chain siloxane polymers are substantially identical. However, in another embodiment, the short chain siloxane polymers may comprise a plurality of different short chain siloxane polymers. For instance, they may differ in chain length (s1), and/or they may differ in (the type of) side groups. Further, they may differ in the type of end group. The short chain siloxane polymers may have (end) groups that are configured to form cross-links upon curing. Note that additionally or alternatively, also one or more side groups per short chain siloxane polymer may be configured to form a cross-link upon curing. For instance, the side groups may include an unsaturated bond comprising group (or a hydrogen group, especially C—H). As can be understood from the above, the short chain siloxane polymers may comprise end groups and/or side groups that are configured to form cross-links upon curing.

Especially, the groups having cross-linking functionality (here individually referring to individual the short chain siloxane polymers, the grafting ligands, and the curable siloxane polymers) are selected from the group consisting of a hydride group and an unsaturated bond comprising group (i.e. a group with at least one unsaturated binding, such as an alkene group, an alkyne group, an imine group, a carbonyl group and an oxime group).

The curable siloxane polymers may in a specific embodiment comprise unsaturated bond comprising groups and hydride groups in a first unsaturated bond hydride group molar ratio, especially being in the range of 3:1-1:3, such as 1:1-1:3. The short chain siloxane polymers may in an embodiment also comprise groups with cross-linking functionality. In a specific embodiment the short chain siloxane polymers comprise unsaturated bond comprising groups and hydride groups in a first unsaturated bond hydride group molar ratio, especially being in the range of 3:1-1:3, such as 1:1-1:1:3. In an embodiment, the curable siloxane polymers comprise unsaturated bond comprising groups and hydride groups in a first unsaturated bond hydride group molar ratio, and the short chain siloxane polymer comprise unsaturated bond comprising groups and hydride groups in a second unsaturated bond hydride group molar ratio, wherein the second unsaturated bond hydride group molar ratio is in the range of 25% to 250%, such as 50-150%, of the first unsaturated bond hydride group molar ratio. The phrase “comprise unsaturated bond comprising groups and hydride groups” and similar phrases especially indicate that the polymers comprise a subset of polymers comprising one or more unsaturated bond comprising groups and a subset of polymers comprising one or more hydride groups.

In a specific embodiment, x1 is at least 40, such as at least 50, especially at least 80. Better and/or more stable systems may then be obtained. In an embodiment, x1 is not larger than 2000, especially not larger than 1,000, such as not larger than 800. In a specific embodiment, x1 is in the range of 40-1,000, such as 40-800, like 100-800. As mentioned above, a combination of different (curable) siloxane grafting ligands may be applied; in such instance x1 may be the (weight) average value. Hence, x1 may especially be in the range of 20-1,000. The larger the x1 value, the easier dispersion of the nano particles may be and the larger the opportunities are to choose siloxanes of various length.

Further, s1 is at least 7, such as especially at least 10, like at least 40, and especially not larger than 400, such as not larger than 200. Hence, s1 may especially be in the range of 7-150. Further, good results may be obtained with x1/s1≥0.80, but in general better results, in the sense of stability and/or transmission (of the wavelength converter) are obtained when x1/s1≥0.95, such as x1/s1≥1.2, like at least 2, such as at least 5. As mentioned above, a combination of different (curable) short chain polymer ligands (S polymers) may be applied; in such instance s1 may be the (weight) average value.

Further, good results may be obtained with y1 being at least 40, such as at least 100, like at least 500. Especially, x1/y1<0.8. Especially, y1 is in the range of 40-2,000, such as 50-1,000. As mentioned above, a combination of different curable siloxane polymers may be applied; in such instance y1 may be the (weight) average value.

Especially, the siloxane grafting ligands and curable siloxane polymers are chemically substantially identical. This may for instance imply that both the siloxane grafting ligands and curable siloxane polymers are polydimethyl siloxanes or polydiphenyl siloxanes or polymethylphenyl (especially 50/50) siloxanes, with the siloxane grafting ligands having at least one side group which is a side group having a grafting functionality. Especially, the short chain siloxane polymers and curable siloxane polymers are chemically substantially identical. This may for instance imply that both the short chain siloxane polymers and curable siloxane polymers are polydimethyl siloxanes or polydiphenyl siloxanes or polymethylphenyl (especially 50/50) siloxanes. Especially, the short chain siloxane polymers and siloxane grafting ligands are chemically substantially identical. This may for instance imply that both the short chain siloxane polymers and siloxane grafting ligands are polydimethyl siloxanes or polydiphenyl siloxanes or polydi methylphenyl (especially 50/50) siloxanes. Hence, in a specific embodiment the short chain siloxane polymers, the siloxane grafting ligands, and the curable siloxane polymers are chemically substantially identical. This may for instance imply that the siloxane grafting ligands, the short chain siloxane polymers, and the curable siloxane polymers are polydimethyl siloxanes or polydiphenyl siloxanes or polymethylphenyl (especially 50/50) siloxanes. As will be clear to the person skilled in the art, also copolymers of two or more of polydimethyl siloxanes, polydiphenyl siloxanes, and polymethylphenyl may be applied.

In a specific embodiment, at least 75%, especially 80%, even more especially 85%, yet even more especially at least 90%, such as especially at least 95% of the side groups of the siloxane grafting ligands and curable siloxane polymers overlap in chemical identity. The overlap in chemical identity can be evaluated by determining the percentages of specific side groups on the siloxane grafting ligands and curable siloxane polymers and counting up the overlapping parts of the percentages. For instance in a hypothetical example, when a siloxane grafting ligand comprises 72% methyl and 25% phenyl side groups, and the curable siloxane polymers comprise 66% methyl and 29% phenyl side groups, than the sum of the overlapping percentages is 66%+25%=91%. Hence, such siloxane grafting ligands and curable siloxane polymers are substantially chemical identical. Especially, at least 75%, especially 80%, even more especially 85%, yet even more especially at least 90%, such as especially at least 95% of the side groups of the short chain siloxane polymers (S polymers) and curable siloxane polymers (Y polymers) overlap in chemical identity. The overlap in chemical identity can be evaluated by determining the percentages of specific side groups on the short chain siloxane polymers and curable siloxane polymers and counting up the overlapping parts of the percentages, similarly as described above. Especially, at least 75%, especially 80%, even more especially 85%, yet even more especially at least 90%, such as especially at least 95% of the side groups of the short chain siloxane polymers (S polymers) and the siloxane grafting ligands (X polymers) overlap in chemical identity. The overlap in chemical identity can be evaluated by determining the percentages of specific side groups on the short chain siloxane polymers and curable siloxane polymers and counting up the overlapping parts of the percentages, similarly as described above. Yet, even more especially, at least 75%, especially 80%, even more especially 85%, yet even more especially at least 90%, such as especially at least 95% of the side groups of the siloxane grafting ligands (X polymers) and curable siloxane polymers (Y polymers) overlap in chemical identity, and at least 75%, especially 80%, even more especially 85%, yet even more especially at least 90%, such as especially at least 95% of the side groups of the short chain siloxane polymers and curable siloxane polymers overlap in chemical identity.

As indicated above, the short chain siloxane polymers and/or the siloxane grafting ligands and/or the curable siloxane polymers may comprise a plurality of different molecules, respectively. In such instance, mean values are used. For instance, assuming a first siloxane grafting ligand having 74% methyl and 22% phenyl side groups and a second siloxane grafting ligand having 70% methyl and 28% phenyl side groups, than the mean percentages are 72% methyl and 25% phenyl. When a combination of different (curable) siloxanes are applied, weight average values may be chosen.

In a specific embodiment, at least 75%, especially 80%, even more especially 85%, yet even more especially at least 90%, such as especially at least 95% of the Si backbone elements (not including terminal groups) of the siloxane grafting ligands have methyl side groups, and especially at least 75%, especially 80%, even more especially 85%, yet even more especially at least 90%, such as especially at least 95% of the Si backbone elements (not including the terminal groups) of the (curable) siloxane polymers have methyl side groups. Hence, in an embodiment, the (solid) siloxane polymer (matrix) and the siloxane grafting ligands comprise poly dimethyl siloxane polymers. Assuming a siloxane comprising 10 silicon backbone units (not including the terminal groups), and 90% methyl side groups, 16 methyl side groups will be present. In a further specific embodiment, at least 60%, yet even more especially at least 70%, such as especially at least 85% of the Si backbone elements (not including terminal groups) of the short chain siloxane polymers have methyl side groups, at least 75%, especially 80%, even more especially 85%, yet even more especially at least 90%, such as especially at least 95% of the Si backbone elements (not including terminal groups) of the siloxane grafting ligands have methyl side groups, and especially at least 75%, especially 80%, even more especially 85%, yet even more especially at least 90%, such as especially at least 95% of the Si backbone elements (not including the terminal groups) of the (curable) siloxane polymers have methyl side groups. Therefore, in an embodiment at least 60% of the Si backbone elements of the short chain siloxane polymers have methyl side groups, wherein at least 90% of the Si backbone elements of the siloxane grafting ligands have methyl side groups and wherein at least 90% of the Si backbone elements of the siloxane polymers have methyl side groups. Especially, in an embodiment, the short chain siloxane polymers, the (solid) siloxane polymer (matrix) and the siloxane grafting ligands comprise poly dimethyl siloxane polymers. In a specific embodiment, the short chain siloxane polymers, the siloxane grafting ligands and the curable siloxane polymers are poly di methyl siloxanes, or poly di phenyl siloxanes, or poly methylphenyl siloxanes.

Especially, the siloxanes for both grafting ligand and the curable siloxane polymers are 100% methyl side groups, or 50/50 methyl/phenyl side groups (with for the siloxane grafting ligands at least one side group is however a side group having grafting functionality, thus such side group is not only methyl or phenyl, but comprises alternatively or additionally for instance an amine or carboxylate). Likewise, even more especially the siloxanes for the short chain siloxane polymers, the grafting ligand and the curable siloxane polymers are 100% methyl side groups, or 50/50 methyl/phenyl side groups. Hence, the invention is not limited to PDMS polymers only. In yet another embodiment, at least 75%, especially 80%, even more especially 85%, yet even more especially at least 90%, such as especially at least 95% of the Si backbone elements (not including the terminal groups) of the siloxane grafting ligands have phenyl side groups and at least 75%, especially 80%, even more especially 85%, yet even more especially at least 90%, such as especially at least 95% of the Si backbone elements (not including the terminal groups) of the siloxane polymers have phenyl side groups. In yet another further embodiment, at least 75%, especially 80%, even more especially 85%, yet even more especially at least 90%, such as especially at least 95% of the Si backbone elements (not including the terminal groups) of the short chain siloxane polymers have phenyl side groups, at least 75%, especially 80%, even more especially 85%, yet even more especially at least 90%, such as especially at least 95% of the Si backbone elements (not including the terminal groups) of the siloxane grafting ligands have phenyl side groups, and at least 75%, especially 80%, even more especially 85%, yet even more especially at least 90%, such as especially at least 95% of the Si backbone elements (not including the terminal groups) of the siloxane polymers have phenyl side groups. As will be clear, the terminal groups (of any of the X, S, and Y polymers) may also comprise methyl, phenyl, or other groups, such as optionally groups having cross-link functionality.

The quantum dots or luminescent nanoparticles, which are herein indicated as wavelength converter nanoparticles, may for instance comprise group II-VI compound semiconductor quantum dots selected from the group consisting of CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, HgS, HgSe, HgTe, CdSeS, CdSeTe, CdSTe, ZnSeS, ZnSeTe, ZnSTe, HgSeS, HgSeTe, HgSTe, CdZnS, CdZnSe, CdZnTe, CdHgS, CdHgSe, CdHgTe, HgZnS, HgZnSe, HgZnTe, CdZnSeS, CdZnSeTe, CdZnSTe, CdHgSeS, CdHgSeTe, CdHgSTe, HgZnSeS, HgZnSeTe and HgZnSTe. In another embodiment, the luminescent nanoparticles may for instance be group III-V compound semiconductor quantum dots selected from the group consisting of GaN, GaP, GaAs, AlN, AlP, AlAs, InN, InP, InAs, GaNP, GaNAs, GaPAs, AlNP, AlNAs, AlPAs, InNP, InNAs, InPAs, GaAlNP, GaAlNAs, GaAlPAs, GaInNP, GaInNAs, GaInPAs, InAlNP, InAlNAs, and InAlPAs. In yet a further embodiment, the luminescent nanoparticles may for instance be I-III-VI2 chalcopyrite-type semiconductor quantum dots selected from the group consisting of CuInS₂, CuInSe₂, CuGaS₂, CuGaSe₂, AgInS₂, AgInSe₂, AgGaS₂, and AgGaSe₂. In yet a further embodiment, the luminescent nanoparticles may for instance be I-V-VI2 semiconductor quantum dots, such as selected from the group consisting of LiAsSe₂, NaAsSe₂ and KAsSe₂. In yet a further embodiment, the luminescent nanoparticles may for instance be a group IV-VI compound semiconductor nano crystals such as SbTe. In a specific embodiment, the luminescent nanoparticles are selected from the group consisting of InP, CuInS₂, CuInSe₂, CdTe, CdSe, CdSeTe, AgInS₂ and AgInSe₂. In yet a further embodiment, the luminescent nanoparticles may for instance be one of the group II-VI, III-V, I-III-V and IV-VI compound semiconductor nano crystals selected from the materials described above with inside dopants such as ZnSe:Mn, ZnS:Mn. The dopant elements could be selected from Mn, Ag, Zn, Eu, S, P, Cu, Ce, Tb, Au, Pb, Tb, Sb, Sn and Tl. Herein, the luminescent nanoparticles based luminescent material may also comprise different types of QDs, such as CdSe and ZnSe:Mn.

It appears to be especially advantageous to use II-VI quantum dots. Hence, in an embodiment the semiconductor based luminescent quantum dots comprise II-VI quantum dots, especially selected from the group consisting of CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, HgS, HgSe, HgTe, CdSeS, CdSeTe, CdSTe, ZnSeS, ZnSeTe, ZnSTe, HgSeS, HgSeTe, HgSTe, CdZnS, CdZnSe, CdZnTe, CdHgS, CdHgSe, CdHgTe, HgZnS, HgZnSe, HgZnTe, CdZnSeS, CdZnSeTe, CdZnSTe, CdHgSeS, CdHgSeTe, CdHgSTe, HgZnSeS, HgZnSeTe and HgZnSTe, even more especially selected from the group consisting of CdS, CdSe, CdSe/CdS and CdSe/CdS/ZnS. In an embodiment, however, Cd-free QDs are applied. In a specific embodiment, the wavelength converter nano-particles comprise III-V QDs, more specifically an InP based quantum dots, such as a core-shell InP—ZnS QDs. Note that the terms “InP quantum dot” or “InP based quantum dot” and similar terms may relate to “bare” InP QDs, but also to core-shell InP QDs, with a shell on the InP core, such as a core-shell InP—ZnS QDs, like a InP—ZnS QDs dot-in-rod.

The luminescent nanoparticles (without coating) may have dimensions in the range of about 1-50 nm, especially 1-20 nm, such as 1-15 nm, like 1-5 nm; especially at least 90% of the nanoparticles have dimension in the indicated ranges, respectively, (i.e. e.g. at least 90% of the nanoparticles have dimensions in the range of 2-50 nm, or especially at least 90% of the nanoparticles have dimensions in the range of 5-15 nm). The term “dimensions” especially relate to one or more of length, width, and diameter, dependent upon the shape of the nanoparticle. In an embodiments, the wavelength converter nanoparticles have an average particle size in a range from about 1 to about 1000 nanometers (nm), and preferably in a range from about 1 to about 100 nm. In an embodiment, nanoparticles have an average particle size in a range from about 1-50 nm, especially 1 to about 20 nm, and in general at least 1.5 nm, such as at least 2 nm. In an embodiment, nanoparticles have an average particle size in a range from about 1 to about 20 nm.

Typical dots may be made of binary alloys such as cadmium selenide, cadmium sulfide, indium arsenide, and indium phosphide. However, dots may also be made from ternary alloys such as cadmium selenide sulfide. These quantum dots can contain as few as 100 to 100,000 atoms within the quantum dot volume, with a diameter of 10 to 50 atoms. This corresponds to about 2 to 10 nanometers. For instance, (spherical) particles such as CdSe, InP, or CuInSe₂, with a diameter of about 3 nm may be provided. The luminescent nanoparticles (without coating) may have the shape of spherical, cube, rods, wires, disk, multi-pods, etc., with the size in one dimension of less than 10 nm. For instance, nanorods of CdSe with the length of 20 nm and a diameter of 4 nm may be provided. Hence, in an embodiment the semiconductor based luminescent quantum dots comprise core-shell quantum dots. In yet another embodiment, the semiconductor based luminescent quantum dots comprise dots-in-rods nanoparticles. A combination of different types of particles may also be applied. For instance, core-shell particles and dots-in-rods may be applied and/or combinations of two or more of the afore-mentioned nano particles may be applied, such as CdS and CdSe. Here, the term “different types” may relate to different geometries as well as to different types of semiconductor luminescent material. Hence, a combination of two or more of (the above indicated) quantum dots or luminescent nano-particles may also be applied.

The term “spherical” may in an embodiment also relate to “quasi spherical”. Note that the nanoparticles are not necessarily (quasi) spherical, they may for instance also have tetrahedrical shapes, etc. Hence, the above-mentioned diameters may in general relate to dimensions. For instance, in the case of tetrahedrical nano particles, the luminescent nanoparticles (without coating) may also have dimensions in the range of about 1-50 nm, especially 1-20 nm, such as 1-15 nm, like 1-5 nm. In such embodiments, the term “dimensions” may also relate to dimensions like length, width height, like the length of a ribbon of a tetrahedron.

One example, such as derived from WO 2011/031871, of a method of manufacturing a semiconductor nanocrystal is a colloidal growth process. Colloidal growth occurs by injection an M donor and an X donor into a hot coordinating solvent. One example of a preferred method for preparing monodisperse semiconductor nanocrystals comprises pyrolysis of organometallic reagents, such as dimethyl cadmium, injected into a hot, coordinating solvent. This permits discrete nucleation and results in the controlled growth of macroscopic quantities of semiconductor nanocrystals. The injection produces a nucleus that can be grown in a controlled manner to form a semiconductor nanocrystal. The reaction mixture can be gently heated to grow and anneal the semiconductor nanocrystal. Both the average size and the size distribution of the semiconductor nanocrystals in a sample are dependent on the growth temperature. The growth temperature necessary to maintain steady growth increases with increasing average crystal size. The semiconductor nanocrystal is a member of a population of semiconductor nanocrystals. As a result of the discrete nucleation and controlled growth, the population of semiconductor nanocrystals that can be obtained has a narrow, monodisperse distribution of diameters. The monodisperse distribution of diameters can also be referred to as a size. Preferably, a monodisperse population of particles includes a population of particles wherein at least about 60%, especially at least 80%, even more especially at 90%, of the particles in the population fall within a specified particle size range. A population of monodisperse particles preferably deviate less than 15% rms (root-mean-square) in diameter and more preferably less than 10% rms and most preferably less than 5%.

In an embodiment, nanoparticles can comprise semiconductor nanocrystals including a core comprising a first semiconductor material and a shell comprising a second semiconductor material, wherein the shell is disposed over at least a portion of a surface of the core. A semiconductor nanocrystal including a core and shell is also referred to as a “core/shell” semiconductor nanocrystal.

For example, the semiconductor nanocrystal can include a core having the formula MX, where M can be cadmium, zinc, magnesium, mercury, aluminum, gallium, indium, thallium, or mixtures thereof, and X can be oxygen, sulfur, selenium, tellurium, nitrogen, phosphorus, arsenic, antimony, or mixtures thereof. Examples of materials suitable for use as semiconductor nanocrystal cores include, but are not limited to, ZnO, ZnS, ZnSe, ZnTe, CdO, CdS, CdSe, CdTe, MgS, MgSe, GaAs, GaN, GaP, GaSe, GaSb, HgO, HgS, HgSe, HgTe, InAs, InN, InP, InSb, AlAs, AIN, AlP, AlSb, TIN, TIP, TlAs, TlSb, PbO, PbS, PbSe, PbTe, Ge, Si, an alloy including any of the foregoing, and/or a mixture including any of the foregoing, including ternary and quaternary mixtures or alloys.

The shell can be a semiconductor material having a composition that is the same as or different from the composition of the core. The shell comprises an overcoat of a semiconductor material on a surface of the core semiconductor nanocrystal can include a Group IV element, a Group II-VI compound, a Group II-V compound, a Group III-VI compound, a Group III-V compound, a Group IV-VI compound, a Group I-III-VI compound, a Group II-IV-VI compound, a Group II-IV-V compound, alloys including any of the foregoing, and/or mixtures including any of the foregoing, including ternary and quaternary mixtures or alloys. Examples include, but are not limited to, ZnO, ZnS, ZnSe, ZnTe, CdO, CdS, CdSe, CdTe, MgS, MgSe, GaAs, GaN, GaP, GaSe, GaSb, HgO, HgS, HgSe, HgTe, InAs, InN, InP, InSb, AlAs, AIN, AlP, AlSb, TIN, TIP, TlAs, TlSb, PbO, PbS, PbSe, PbTe, Ge, Si, an alloy including any of the foregoing, and/or a mixture including any of the foregoing. For example, ZnS, ZnSe or CdS overcoatings can be grown on CdSe or CdTe semiconductor nanocrystals. An overcoating process is described, for example, in U.S. Pat. No. 6,322,901. By adjusting the temperature of the reaction mixture during overcoating and monitoring the absorption spectrum of the core, over coated materials having high emission quantum efficiencies and narrow size distributions can be obtained. The overcoating may comprise one or more layers. The overcoating comprises at least one semiconductor material which is the same as or different from the composition of the core. Preferably, the overcoating has a thickness from about one to about ten monolayers. An overcoating can also have a thickness greater than ten monolayers. In an embodiment, more than one overcoating can be included on a core.

In an embodiment, the surrounding “shell” material can have a band gap greater than the band gap of the core material. In certain other embodiments, the surrounding shell material can have a band gap less than the band gap of the core material. In an embodiment, the shell can be chosen so as to have an atomic spacing close to that of the “core” substrate. In certain other embodiments, the shell and core materials can have the same crystal structure. Examples of semiconductor nanocrystal (core)shell materials include, without limitation: red (e.g., (CdSe)ZnS (core)shell), green (e.g., (CdZnSe)CdZnS (core)shell, etc.), and blue (e.g., (CdS)CdZnS (core)shell (see further also above for examples of specific wavelength converter nanoparticles, based on semiconductors.

Hence, the above-mentioned outer surface may be the surface of a bare quantum dot (i.e. a QD not comprising a further shell or coating) or may be the surface of a coated quantum dot, such as a core-shell quantum dot (like core-shell or dot-in-rod), i.e. the (outer) surface of the shell. The grafting ligand thus especially grafts to the outer surface of the quantum dot, such as the outer surface of a dot-in-rod QD.

Therefore, in a specific embodiment, the wavelength converter nanoparticles are selected from the group consisting of core-shell nano particles, with the cores and shells comprising one or more of CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, HgS, HgSe, HgTe, CdSeS, CdSeTe, CdSTe, ZnSeS, ZnSeTe, ZnSTe, HgSeS, HgSeTe, HgSTe, CdZnS, CdZnSe, CdZnTe, CdHgS, CdHgSe, CdHgTe, HgZnS, HgZnSe, HgZnTe, CdZnSeS, CdZnSeTe, CdZnSTe, CdHgSeS, CdHgSeTe, CdHgSTe, HgZnSeS, HgZnSeTe, HgZnSTe, GaN, GaP, GaAs, AlN, AlP, AlAs, InN, InP, InAs, GaNP, GaNAs, GaPAs, AlNP, AlNAs, AlPAs, InNP, InNAs, InPAs, GaAlNP, GaAlNAs, GaAlPAs, GaInNP, GaInNAs, GaInPAs, InAlNP, InAlNAs, and InAlPAs. In general, the cores and shells comprise the same class of material, but essentially consist of different materials, like a ZnS shell surrounding a CdSe core, etc.

As indicated above, these type of wavelength converter nanoparticles may be embedded in a polymerix matrix, the siloxane polymer matrix. This is in general a solid polymer (i.e. not liquid), though the polymer may in an embodiment be flexible and may in an embodiment also have gel-like properties. Herein, the term “solid polymer” is used, as to indicate that the polymeric end product of the process of the invention is not a liquid or a solved polymer, but a tangible product (at room temperature (and atmospheric pressure)) in the form of for instance particles, a film, a plate, etc. Hence, in an embodiment, the wavelength converter is selected from the group consisting of a coating, a self-supporting layer, and a plate; which wavelength converter is thus especially solid at room temperature, especially even up to 100° C., especially even up to 150° C., especially even up to 200° C.). The wavelength converter may be flexible or may be rigid. Further, the wavelength converter may be flat or curved (in one or two dimensions). Further, optionally the wavelength converter may comprise outcoupling structures at at least part of the external surface of the wavelength converter.

The process of the invention at least comprises two process elements, which will in general be executed consecutively, with the first process element preceding the second process element. The fact that two process elements are explicitly mentioned, does not exclude the presence of one or more other process elements, which may be included in the process before the first process element, and/or between the first and the second process element, and/or after the second process element. For instance, the process of the invention may also include an exchange of existing grafting molecules on the quantum nano particle with grafting molecules as defined in the present invention. This process may further optionally include removal of excess ligands (i.e. ligands that are not bound to wavelength converter nano particles.

The first process element includes the mixing of the grafted nano particles (i.e. the wavelength converter nano particles having an outer surface grafted with the siloxane grafting ligands) and the curable siloxane polymers. The grafted nano particles are especially available in a liquid comprising short chain siloxane polymers; this combination is indicated as “first liquid”.

In general, the wavelength converter nano particles are commercially available in a (first) solvent, such as toluene, and with ligands which are no siloxanes (such as an oleate (9z-octadec-9-enoelate) and/or TOPO (tri-octyl phosphine oxide)). These ligands have to be exchanged with the grafting ligands as described herein. Hence, in an embodiment the grafted nano particles (as described herein) are obtainable by mixing a nano particle comprising liquid (especially said commercially available liquid of nano particles with ligands (which are no siloxanes) with a liquid comprising grafting ligands. This liquid comprising grafting ligands may comprise a solvent, but may especially substantially comprise the grafting ligands per se. Optionally, these grafting ligands are curable. By combining the nano particles and the grafting ligands, the grafting ligands attach to the external surface of the nano particles (they may exchange for ligands of the nanoparticles in the nano particle comprising liquid (in a (first) solvent).

The thus obtained grafted nano particles are especially separated from the mixture of solvent(s) and remaining ligands. For instance, the grafted nano particles may be precipitated and washed, e.g. to remove unbound grafting ligands. Thereafter, it may be redispersed in a second solvent, such as toluene. Now, the nano particles are grafted with the desired ligands and available in a liquid. This is called the starting liquid. This starting liquid can then be combined with a liquid comprising the short chain siloxane polymers. This liquid comprising the short chain siloxane polymers may comprise a solvent, but may especially substantially comprise the short chain siloxane polymers per se. Especially, these short chain siloxane polymers are curable. The combination of these liquids leads to a dispersion of the grafted ligands in the short chain siloxane polymers. The thus obtained liquid is indicated as first liquid, and comprises the grafted nano particles dispersed in the short chain siloxane polymers. As x1/s1≥0.8, especially >1, the wavelength converter nano particles with grafting ligands are relatively easily miscible with the short chain siloxane polymers.

Remaining solvent is preferably removed, such as by one or more of evaporation (under reduced pressure) or with an adsorbent. Other techniques may be applied as well to substantially remove the remaining solvent, which essentially stems from the first liquid. Hence, it is especially desired that when solvent is available in the first liquid, before curing, and especially before combining (mixing) the first liquid with the curable siloxane polymers, at least part of the solvent is removed, preferably substantially all solvent. In a specific embodiment, the first liquid may comprise one or more solvents having a boiling point up to a maximum 120° C., especially up to 150° C., with a total amount of the solvents of not more than 2 wt. %, especially not more than 1 wt. %. Hence, the method of the invention may also include removing solvent from the first liquid at least until a total amount of the solvents of not more than 2 wt. % is reached. Hence, the QDs may be dispersed in the short chain siloxane polymers, i.e. the first liquid, which may comprise a solvent in a low amount or substantially no solvent at all. Toluene, for instance, has a boiling point of about 111° C., heptane of about 99° C., and hexamethyldisiloxane of about 101° C. Hence, in an embodiment solvents having a boiling point below 120° C. are not available (including after removal) in the first liquid (before curing in the presence of curable siloxane polymers) or in the wavelength converter or only available in a total amount of not more than 2 wt. %, especially not more than 1 wt. %.

Herein, a solvent is considered to be a solvent when at room temperature at least 0.1 gram/l of a species to be solved can be solved in the solvent. The solvent could be any common, preferably non-polar, solvents with preferably a boiling point lower than 150° C., such as lower than 120° C. For instance, the solvent could be toluene, benzene, hexane, cyclohexane, etc. The solvent could be a polar solvent. For instance, the solvent could be chloroform, acetone, acetone nitrile, ethyl acetate, butyl acetate, petroleum ether, etc. Mixing may be done with conventional techniques. Optionally, the mixture may be heated.

Further, the first liquid, i.e. liquid comprising the grafted nano particles dispersed in the short chain (curable) siloxane polymers (S polymers) is combined with the curable siloxane polymers (Y polymers). The thus obtained combination can be cured, or can be provided on a substrate or in a cavity, and then be cured, etc. Curing may be done with techniques known in the art. As indicated above, to this end at least part of the curable siloxane polymers may have reactive groups that are configured to form cross-links upon curing. Curing may be assisted by a catalyst, such as especially a Pt catalyst. Further, the mixture may be heated and/or irradiated to initiate and/or propagate curing. By curing, a (solid) matrix or host for the grafted wavelength converter nano particles is obtained (the latter being embedded and distributed in the former).

Hence, the invention further provides a process including (a) mixing (i) a first liquid comprising (i1) short chain siloxane polymers and (i2) wavelength converter nano particles having an outer surface grafted with siloxane grafting ligands and (ii) curable siloxane polymers, wherein the process further includes removing a solvent (i.e. any remaining solvent) from the first liquid, especially until an amount equal to or lower than 2 wt. %, especially equal to or lower than 1 wt. %, even more especially equal to or lower than 0.5 wt. % is obtained, before curing (the mixture of the first liquid and curable siloxane polymers), and especially before mixing the first liquid and the curable siloxane polymers.

As suggested above, the process of the invention may provide a (luminescent) wavelength converter comprising a (solid) polymer within the polymer article embedded wavelength converter nano particles having an outer surface grafted with grafting molecules. As indicated above, the (luminescent) wavelength converter may for instance be transparent or translucent, especially substantially transparent.

The process of the invention may lead in an embodiment to a product wherein at least part of the grafting ligand is interwoven (via e.g. cross-links) with siloxane polymer(s). The latter embodiment may be the case when the siloxane grafting ligands may be able to react with the curable (or cured) siloxane polymers. Hence, in an embodiment, one or more siloxane grafting ligands may comprise functional groups that are configured to form cross-links with especially the curable siloxane polymers (that form the host material or matrix). In an embodiment, the process of the invention may lead to a product wherein at least part of the short chain siloxane polymers are interwoven (via e.g. cross-links) with siloxane polymer(s). The latter embodiment may be the case when the short chain siloxane polymers may be able to react with the curable (or cured) siloxane polymers. Hence, in an embodiment, one or more short chain siloxane polymers may comprise functional groups that are configured to form cross-links with especially the curable siloxane polymers (that (substantially) form the host material or matrix). In a further embodiment, the process of the invention may (also) lead in an embodiment to a product wherein at least part of the grafting ligand is interwoven (via e.g. cross-links) with short chain siloxane polymers. The latter embodiment may be the case when the siloxane grafting ligands may be able to react with the short chain siloxane polymers. Hence, in an embodiment, one or more siloxane grafting ligands and one or more short chain siloxane polymers may comprise functional groups that are configured to form cross-links with each other. Hence, in a specific embodiment at least part of the second type of siloxane polymers are cross-linked, also at least part of the first type of siloxane polymers are cross-linked, and at least part of the first type of siloxane polymers and the second type of siloxane polymers are cross-linked with each other.

The process of the invention may thus also provide the herein defined wavelength converter (or wavelength converter element) with wavelength converter nano particles embedded therein. Hence, the herein defined wavelength converter (or wavelength converter element), with wavelength converter nano particles embedded therein, is in an embodiment obtainable by the process as defined herein. Therefore, as indicated above, in a further aspect, the invention provides a wavelength converter comprising a siloxane polymer matrix with wavelength converter nano particles embedded therein, wherein: (a) the wavelength converter nano particles have an outer surface grafted with siloxane grafting ligands (X polymers), and (b) the siloxane polymer matrix comprises siloxane polymers of a first type (S polymers or S polymer based) and siloxane polymers of a second type (Y polymers or Y polymer based), wherein at least part of the second type of siloxane polymers are cross-linked; wherein the siloxane polymers of the first type comprise short chain siloxane polymers having s1 Si backbone elements, wherein the siloxane grafting ligands comprise siloxane grafting ligands having x1 Si backbone elements, wherein the siloxane polymers of the second type comprise siloxane polymers having y1 Si backbone elements; wherein x1/s1≥0.8, such as x1/s1≥0.95, such as >1, like at least ≥1.2, such as at least >2, wherein s1<y1, such as s1/y1<0.1, and wherein in a specific embodiment x1<y1.

Especially, the wavelength converter comprises 0.001-25 wt. % wavelength converter nanoparticles relative to the total weight of the wavelength converter, such as 0.1-20 wt. %, especially not more than 5 wt. %, such as 0.1-2 wt. %. Further, in a specific embodiment, s1<x1<y1. Especially, s1 is at least 7, x1 is at least 20, and y1 is at least 100, such as in the range of 100-1,000.

As indicated above, during the production of the wavelength converter especially solvent may be removed (especially before curing). Hence, in an embodiment the wavelength converter does not comprise a solvent. However, would the wavelength converter still comprise some solvent, the one or more remaining solvents may be present in a total amount of the one or more solvents of not more than 2 wt. %, especially below 1 wt. % (sum of the one or more solvents) relative to the weight of the wavelength converter.

As indicated above, the short chain polymers may also be curable. Hence, a matrix in the form of cross-linked network may be obtained, with Y polymers being interconnected, S polymers being interconnected and S and Y being interconnected. When the grafting ligands comprise curable groups, also these may form part of the network.

Especially, the matrix of the cured (curable) siloxane polymers is transmissive for light having a wavelength selected from the range of 380-750 nm. For instance, the matrix of the cured (curable) siloxane polymers may be transmissive for blue, and/or green, and/or red light. Especially, the matrix of the cured (curable) siloxane polymers is transmissive for at least the entire range of 420-680 nm. Especially, the matrix of the cured (curable) siloxane polymers has a light transmission in the range of 50-100%, especially in the range of 70-100%, for light generated by the light source of the lighting unit (see also below) and having a wavelength selected from the visible wavelength range. In this way, the matrix of the cured (curable) siloxane polymers is transmissive for visible light from the lighting unit. The transmission or light permeability can be determined by providing light at a specific wavelength with a first intensity to the material and relating the intensity of the light at that wavelength measured after transmission through the material, to the first intensity of the light provided at that specific wavelength to the material (see also E-208 and E-406 of the CRC Handbook of Chemistry and Physics, 69th edition, 1088-1989). The wavelength converter may be transparent or translucent, but may especially be transparent. Especially, the wavelength converter is substantially transparent and/or does not substantially scatter light. When the wavelength converter is transparent, light of the light source may not entirely be absorbed by the wavelength converter. Especially when using blue light, this may be of interest, as the blue light may be used to excite the wavelength converter nano particles and may be used to provide a blue component (in white light). Hence, especially curable siloxane polymers are applied that provide a substantially transmissive matrix (or host) for the wavelength converter nano particles.

Further, as stated above, the invention also provides a lighting unit comprising (i) a light source configured to generate light source light and (ii) a wavelength converter configured to convert at least part of the light source light into converter light, wherein the wavelength converter comprises the solid polymer obtainable according to the process as defined herein or the wavelength converter as defined herein. Hence, in a further aspect, the invention provides a lighting device comprising: a light source configured to generate light source light, a wavelength converter obtainable by the process as defined herein or the wavelength converter per se as defined herein, configured to convert at least part of the light source light into visible converter light.

It may be advantageous, in view of efficiency and/or stability, to arrange the nano particles, or especially the wavelength converter, at a non-zero distance, such as 0.5-50 mm, like 1-50 mm, from the light source. Hence, in an embodiment, the wavelength converter may be configured at a non-zero distance of the light source. For instance, the wavelength converter, or especially the luminescent material, may be applied to or may be comprised by a window of the lighting unit. In case the light source is configured to provide blue light, the luminescent material may be configured to convert only part of the light source light. The blue light of the light source and the luminescent material light of the wavelength converter nano particles based luminescent material together may in an embodiment provide white lighting unit light. Hence, in an embodiment, the wavelength converter is configured at a non-zero distance from the light source. Note however that the invention is not limited to applications wherein the distance between the wavelength converter and the light source is non-zero. The invention, and the herein described specific embodiments, may be also applied in other embodiments wherein the light source and wavelength converter are in physical contact. In such instances, the wavelength converter may especially be configured in physical contact with e.g. a LED die.

In a further embodiment, the light source comprise a solid state light source, such as a solid state light emitting device or solid state laser. The term light source may also relate to a plurality of light sources. As indicated above, the lighting device might be applied as back lighting unit in an LCD application. Hence, the invention provides in a further aspect a liquid crystal display device comprising a back lighting unit, wherein the back lighting unit comprises one or more lighting devices as defined herein.

In addition to the wavelength converter nano particles, the wavelength converter may comprise host other (particulate) materials, such as for instance one or more of an organic dye and reflective (non luminescent) particles, like TiO₂. Such (particulate) material(s) may be mixed with the wavelength converter nano particles and the curable siloxane polymers. Hence, the phrase “mixing (i) wavelength converter nano particles (herein also indicated as “nano particles”) with an outer surface grafted with siloxane grafting ligands and (ii) curable siloxane polymers” may also relate to “mixing (i) wavelength converter nano particles (herein also indicated as “nano particles”) with an outer surface grafted with siloxane grafting ligands and (ii) curable siloxane polymers, and optionally one or more other materials”.

Hence, amongst others, it was found that when the QDs are first stabilized in low-viscosity PDMS melt and subsequently added to a higher molecular weight pdms or a commercially available silicone, the mixture can remain non scattering. In addition it was found an effect on the presence of solvent: if the solvent in which the quantum dots are first dispersed (e.g. toluene) is still present in the mixture of low-viscosity PDMS it remains clear for a limited time when added to high viscosity PDMS, ranging from minutes to hours, while if the solvent is removed mixtures remain can remain clear for a long time (in some cases for a limited amount of time, ˜1 hour, in some case for at least weeks), for which we will show specific examples. This implies that commercial silicones used within this invention are ideally solvent-free. It was also found that when the low molecular weight pdms is replaced by a vinyl and/or hydride containing, for instance a vinyl or hydride terminated low molecular pdms, the ligand exchanged quantum dots remain colloidally stable. It can therefore be envisaged that silicone compatible quantum dots can be commercially supplied in a melt or concentrated solution of low molecular weight potentially cross-linkable pdms to be mixed with other high molecular weight (commercial) silicones to yield quantum dots containing silicone materials in which the quantum dots are well dispersed. The examples below illustrate specific embodiments, all prepared with ligand exchanged quantum dots. The examples below show that for some compositions the formation of aggregates in the mixtures can be fairly slow. Fast curable silicones (eg UV-curable) silicones could therefore be favorable for this application. Another logic consequence is that the mixing of QDs with the commercial silicone is preferably (but not exclusively) done just prior to curing, not far in advance.

In addition to the quantum dots, also another luminescent material may be applied. The term “other luminescent material” may also relate to a plurality of (different) luminescent materials. The other luminescent material may be contained in the matrix as well. Alternatively or additionally, the other luminescent material may be present as coating on the light converter. Alternatively or additionally, the luminescent material may be arranged anywhere else in the lighting device. Also the other luminescent material may especially be configured to convert at least part of the light source light into visible converter light. Especially, the other luminescent material is configured to provide one or more of blue, green, yellow and orange light, more especially at least one or more of green and yellow light. Several options are possible, including (Ca,Sr,Ba)(Al,Ga,In)₂(O,S,Se)₄:Eu²⁺, a thiogallate, especially such luminescent material at least comprising Sr, Ga and S, such as SrGa₂S₄:Eu²⁺. These type of luminescent materials may especially be narrow band green emitters. Optionally or alternatively, the other luminescent material may comprise a M₃A₅O₁₂:Ce³⁺ (garnet material), wherein M is selected from the group consisting of Sc, Y, Tb, Gd, and Lu, wherein A is selected from the group consisting of Al and Ga. Preferably, M at least comprises one or more of Y and Lu, and wherein A at least comprises Al. These types of materials may give highest efficiencies. Embodiments of garnets especially include M₃A₅O₁₂ garnets, wherein M comprises at least yttrium or lutetium and wherein A comprises at least aluminum. Such garnet may be doped with cerium (Ce), with praseodymium (Pr) or a combination of cerium and praseodymium; especially however with at least Ce. Additionally or alternatively, the other luminescent material may comprise one or more of selected from the group consisting of divalent europium containing nitride luminescent material or a divalent europium containing oxonitride luminescent material, such as one or more materials selected from the group consisting of (Ba,Sr,Ca)S:Eu, (Mg,Sr,Ca)AlSiN₃:Eu and (Ba,Sr,Ca)₂Si₅N₈:Eu. In these compounds, europium (Eu) is substantially or only divalent, and replaces one or more of the indicated divalent cations. In general, Eu will not be present in amounts larger than 10% of the cation, especially in the range of about 0.5-10%, more especially in the range of about 0.5-5% relative to the cation(s) it replaces. The term “:Eu” or “:Eu²”, indicates that part of the metal ions is replaced by Eu (in these examples by Eu²). For instance, assuming 2% Eu in CaAlSiN₃:Eu, the correct formula could be (Ca_(0.98)Eu_(0.02))AlSiN₃. Divalent europium will in general replace divalent cations, such as the above divalent alkaline earth cations, especially Ca, Sr or Ba. The material (Ba,Sr,Ca)S:Eu can also be indicated as MS:Eu, wherein M is one or more elements selected from the group consisting of barium (Ba), strontium (Sr) and calcium (Ca); especially, M comprises in this compound calcium or strontium, or calcium and strontium, more especially calcium. Here, Eu is introduced and replaces at least part of M (i.e. one or more of Ba, Sr, and Ca). Further, the material (Ba,Sr,Ca)₂Si₅N₈:Eu can also be indicated as M₂Si₅N₈:Eu, wherein M is one or more elements selected from the group consisting of barium (Ba), strontium (Sr) and calcium (Ca); especially, M comprises in this compound Sr and/or Ba.

The term white light herein, is known to the person skilled in the art. It especially relates to light having a correlated color temperature (CCT) between about 2000 and 20000 K, especially 2700-20000 K, for general lighting especially in the range of about 2700 K and 6500 K, and for backlighting purposes especially in the range of about 7000 K and 20000 K, and especially within about 15 SDCM (standard deviation of color matching) from the BBL (black body locus), especially within about 10 SDCM from the BBL, even more especially within about 5 SDCM from the BBL. The terms “violet light” or “violet emission” especially relates to light having a wavelength in the range of about 380-440 nm. The terms “blue light” or “blue emission” especially relates to light having a wavelength in the range of about 440-490 nm (including some violet and cyan hues). The terms “green light” or “green emission” especially relate to light having a wavelength in the range of about 490-560 nm. The terms “yellow light” or “yellow emission” especially relate to light having a wavelength in the range of about 540-570 nm. The terms “orange light” or “orange emission” especially relate to light having a wavelength in the range of about 570-600. The terms “red light” or “red emission” especially relate to light having a wavelength in the range of about 600-750 nm. The term “pink light” or “pink emission” refers to light having a blue and a red component. The terms “visible”, “visible light” or “visible emission” refer to light having a wavelength in the range of about 380-750 nm.

The term “substantially” herein, such as in “substantially all emission” or in “substantially consists”, will be understood by the person skilled in the art. The term “substantially” may also include embodiments with “entirely”, “completely”, “all”, etc. Hence, in embodiments the adjective substantially may also be removed. Where applicable, the term “substantially” may also relate to 90% or higher, such as 95% or higher, especially 99% or higher, even more especially 99.5% or higher, including 100%. The term “comprise” includes also embodiments wherein the term “comprises” means “consists of”. The term “and/or” especially relates to one or more of the items mentioned before and after “and/or”. For instance, a phrase “item 1 and/or item 2” and similar phrases may relate to one or more of item 1 and item 2.

Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein.

The devices herein are amongst others described during operation. As will be clear to the person skilled in the art, the invention is not limited to methods of operation or devices in operation.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb “to comprise” and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article “a” or “an” preceding an element does not exclude the presence of a plurality of such elements. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

The invention further applies to a device comprising one or more of the characterizing features described in the description and/or shown in the attached drawings. The invention further pertains to a method or process comprising one or more of the characterizing features described in the description and/or shown in the attached drawings.

The various aspects discussed in this patent can be combined in order to provide additional advantages. Furthermore, some of the features can form the basis for one or more divisional applications.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:

FIGS. 1a-1c schematically depict some aspects of the device(s) of the invention;

FIGS. 2a-2e schematically depict some further aspects of the invention;

FIGS. 3a-3j schematically depict some further aspects of siloxanes and some further aspects of the invention.

The drawings are not necessarily on scale.

FIGS. 4a and 4b show TEM pictures of QDs before and after exchange of siloxane grafting ligands according to the invention, respectively.

FIG. 5a-5b show photographs of a cross-linked silicone comprising QDs after exchange of siloxane grafting ligands according to the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 1a schematically depicts a lighting device 1 comprising a light source 10 configured to generate light source light 11 and a wavelength converter 100 configured to convert at least part of the light source light 11 into visible converter light 121. Here schematically only one light source 10 is depicted. However, more than one light source 10 may be present. The wavelength converter has an upstream side 101 (part of an external surface of the wavelength converter), which is at least partly directed to the light source 10, and a downstream side 102 (part of an external surface of the wavelength converter), which faces away from the light source 10 (in this transmissive configuration). The wavelength converter 100 comprises a polymeric host material 110 with wavelength converter nanoparticles 120 embedded in the polymeric host material 110. These can be dots, rods, a combination thereof, etc. (see also above). The wavelength converter nanoparticles 120 generate upon excitation by the light source light 11 visible converter light (and optionally also non-visible radiation, like IR radiation). At least part of the converter light 121 escapes from the downstream side 102 as lighting device light 5. This lighting device light 5, of which at least part is in the visible, at least contains part of the converter light 121, and may optionally also contain some remaining light source light 11. Especially, the light source light is blue light. The light converter nano particles or wavelength converter nanoparticles 120 may be chosen to provide white light upon conversion of the light of the light source. Alternatively or additionally, the wavelength converter nanoparticles 120 are chosen to provide light upon conversion of the light of the light source and this light together with the light of the light source provides white light. Alternatively or additionally, the light converter nano particles or wavelength converter nanoparticles 120 may be chosen to provide, together with another luminescent material, white light upon conversion of the light of the light source. Alternatively or additionally, the wavelength converter nanoparticles 120 are chosen to provide, together with another luminescent material, light upon conversion of the light of the light source and this light together with the light of the light source provides white light. The other luminescent material may also refer to a plurality of luminescent materials. Examples are mentioned above. FIG. 1a schematically depicts the lighting device in operation.

FIG. 1b schematically depicts another embodiment, wherein the wavelength converter 100 is encapsulated. An encapsulation 400 encloses the wavelength converter; this encapsulation may substantially block oxygen (and/or H₂O) transporter from the atmosphere to the wavelength converter. This may add to the stability of the wavelength converter nanoparticles 120 (and the polymeric host). The combination of wavelength converter 100 and encapsulation 400 is herein also indicated as wavelength converter unit 1100.

FIG. 1c schematically depicts one of the applications of the lighting unit 1, here in a liquid crystal display device 2, which comprises a back lighting unit 200 which comprises one or more lighting units 1 (here, one lighting unit is schematically depicted), as well as a LCD panel 300, which can be backlighted with the lighting device light 5 of the lighting unit(s) 100 of the back lighting unit 200.

The converter 100 may especially be arranged at a non-zero distance d from the light source 10, which may for instance be a light emitting diode, although the distance d may also be zero, for instance when the luminescent material 30 is applied on a LED die or embedded in a (silicone) cone on the LED die. The converter may optionally allow at least part of the light source light 21 penetrate through the converter. In this way, downstream of the converter, a combination of converter light 121 and light source light 11 may be found. The light downstream of the wavelength converter is indicated a lighting device light 5. The distance d may especially be in the range of 0.1-100 mm, especially 0.5-100 mm, such as 1-20 mm, like especially 1-50 mm, like about 1-3 for applications close by the light source and 5-50 mm for more remote applications. Note however that the invention is not limited to applications wherein d>0. The invention, and the herein described specific embodiments, may be also applied in other embodiments with d=0. In such instances, the wavelength converter may especially be configured in physical contact with the LED die.

In addition to the ligand grafted semiconductor based wavelength converter nano particles 120, the wavelength converter 100 may optionally comprise also other types of luminescent materials, for instance to tune the color of the lighting unit light 5, to increase the color rendering, to tune the color temperature, etc.

The terms “upstream” and “downstream” relate to an arrangement of items or features relative to the propagation of the light from a light generating means (here the especially the first light source), wherein relative to a first position within a beam of light from the light generating means, a second position in the beam of light closer to the light generating means is “upstream”, and a third position within the beam of light further away from the light generating means is “downstream”.

FIG. 2a schematically depict a non-limiting number of examples of wavelength converter nano particles 120, here quantum dots (QDs). By way of example, (i) indicates a QD particle, having the shape of a dot, without further layers. For instance, this may be e.g. CdSe. Siloxane grafting ligands are not shown for the sake of clarity (see below). The QD example (ii) schematically depicts a core shell system, by way of example (CdSe)ZnS (core)shell. The QD example (iii) schematically depicts a dot in rod QD system, e.g. (CdS)ZnS dot in rod (which is also a type of core-shell QD). The wavelength converter nano particles have an outer surface indicated with reference 127.

FIG. 2b schematically depicts a QD with grafting ligand. As can be seen, in this example the side groups having a grafting functionality (the group is indicated with reference 131) are not end groups; the grafting ligands attach to the outer surface 127 of the wavelength converter nano particles. Hence, the siloxane grafting ligands may have (at least) two tails. Would the siloxane grafting ligands have more than one side group having a grafting functionality, other (more complex) structures may be found. In the insert, an enlargement is shown, with two tails at both sides of the silicone backbone element that bears the side group having a grafting functionality that binds to the outer surface 127 of the wavelength converter nano particle. The grafting ligands are indicated with reference 130.

FIG. 2c schematically depicts wavelength converter nano particles 120 in a liquid comprising short chain siloxane polymers, which are indicated with reference 230; the liquid is herein also indicated as first liquid. Reference 231 refers to this liquid. This first liquid 231 is combined with curable siloxane polymers (see FIG. 2d ). To this end, the mixture obtained in FIG. 2c , i.e. the grafted wavelength converter particles in short chain siloxane polymers is combined with a liquid 31 comprising curable siloxane polymers.

FIG. 2d very schematically depicts the situation before curing, wherein for instance the wavelength converter nano particles with siloxane grafting ligands bound to the outer surface of the nano particles as well as uncured siloxane polymers, indicated with reference 330, and short chain siloxane polymers 230, are distributed in a liquid in the vessel, which is indicated with reference 300, having a vessel wall (including bottom) indicated with reference 310. The liquid is indicated with reference 31; it may be used to disperse the grafted nano particles 120; further, it may be applied to dissolve the curable siloxane polymers.

After curing, such as by heating and in the presence of a catalyst (for the sake of clarity not depicted) in the mixture in the vessel, the cured system is obtained, i.e. the wavelength converter, as schematically depicted in FIG. 2e . The cured siloxane polymers, now forming a silicone, are considered the polymeric host material 110 for the wavelength converter nano particles 120, such as QDs (see also above).

FIGS. 3a-3h are discussed further below in the course of the experimental part.

FIG. 3i very schematically depicts part of a siloxane polymer, with only 6 silicone backbone elements depicted; Si backbone elements 1 and 6 are terminal groups, with e.g. terminal end groups R₃′-R₃′″ and end groups R₄′-R₄′″, respectively. These terminal end groups may comprise e.g. (one or more of) OH, methyl, phenyl, vinyl, hydrogen, etc. etc., as known in the art. Each (non-terminal) silicone backbone element has two side groups, which can in principle all be different, but which can also all be the same, except for the fact that the grafting ligand has at least one side group having a grafting functionality as defined above.

FIG. 3j is shown to indicate how the chemical overlap can be determined. As indicated above, especially at least 75%, especially 80%, even more especially 85%, yet even more especially at least 90%, such as especially at least 95% of the side groups of the siloxane grafting ligands and curable siloxane polymers overlap in chemical identity. R3′-R3″, R4′-R4′″, R5′-R5″ and R6′-R6′″ are end groups, and are not considered when evaluating the chemical overlap of the side groups. FIG. 3j schematically shows simple (but common) systems with only two different side groups. The grafting ligand has by way of example 4 non-terminal silicone backbone elements (and two terminal Si backbone elements); the curable siloxane polymer has by way of example 3 silicone backbone elements (and two terminal Si backbone elements). The grafting ligand has by way of example one side group having a grafting functionality (FSG). In below table, the overlap in chemical identity is displayed. Note that in columns II and V only number of the non-terminal Si backbone elements is given, as only these can provide side groups; the terminal Si backbone elements have end groups.

II III IV V VI VII VIII IX I No Si. No. R1 % R1 No. R2 % R2 FSG % FSG Sum % Grafting 4 5 =5/(2*4) * 100% = 2 =2/(2*4) * 100% = 1 =1/(2*4) * 100% = 100% ligand 62.5% 25% 12.5% Curable siloxane 3 5 =5/(2*3) *100% = 1 =1/(2*3) * 100% = 0 0 100 polymer 83.3% 16.7% Overlap 62.5% 16.7 0 79.2 Hence, here the chemical overlap is 79.2%. In practice, the chemical overlap will in general be higher, such as at least 90%. Likewise, this methodology applies for calculating the chemical overlap between short chain siloxane polymers and curable siloxane polymers.

EXPERIMENTAL Experimental Results Quantum Dots in Silicones Examples of Ligands with Various Sizes and Functional Groups

In our search for compatible QD-ligand/silicone systems we have systematically investigated up to 20 ligands of different composition/functionality, and up to 10 different silicones. Roughly, the ligands can be classified as: Short monofunctional siloxane (e.g. 3-Aminopropylpentamethyldisiloxane, AB129258; supplier ABCR); see FIG. 3a ; Short bifunctional siloxane (e.g. 1,3-Bis(3-aminopropyl)tetramethyldisiloxane, AB 110832); see FIG. 3b ; Endfunctionalized PDMS with amine or acid on both ends (“symmetric”, e.g. Polydimethylsiloxane, aminopropyl terminated, AB111872); see FIG. 3c ; End functionalized PDMS with amine or acid on one end (“asymmetric”, e.g. Mono carboxydecyl terminated polydimethylsiloxane, asymmetric, AB 252409); see FIG. 3d ; Side-functionalized PDMS with amine or acid (2-3% Aminopropylmethylsiloxane)-dimethyl siloxane copolymer, AB109373); see FIG. 3e , with reference 131 referring to the side group having a grafting functionality.

In all cases, a small amount of QDs in toluene was added to 1 ml of the pure ligand (though solutions may also be used), which gave very turbid mixtures is nearly all cases. The mixture was stirred for more than 12 hours at 100° C. In most cases, the pure ligand-QD mixture became transparent within an hour, which provides evidence for ligand exchange. After cooling, this mixture was directly added to the various silicones, and the degree of mixing was determined by visual inspection of the obtained mixture. In contrast to experiments in examples further on in this document, the QDs were not purified from the excess ligands in the experiments described here. An overview of all ligands tested can be found in the table below. It was observed that only QDs with side-functionalized ligands (the last category) gave improved miscibility with the selected silicones. It was later found that ligands very similar to AB109373 (AB109374, AB124359, AB 116669 (see table) (latter two are branched)) showed similar transparent mixtures. The common factor is that it involves PDMS chains with on average 1 amine group (randomly distributed) in the side chain, and with a molecular weight of ˜5000-1000, corresponding to a viscosity of 100-300 cSt. An illustration of these molecules is given in FIGS. 3f and 3g . From the experiments above it appears that short ligands, and/or end-functionalized ligands did not show an improved miscibility with silicones. The effect of long versus short ligands is explained in more detail below. For that purpose, we have tested once again two long-chain end-functionalized siloxane ligands (the AB109371 and 153374 with viscosity 100 and 1000 cSt respectively), but it was found that the ligand exchange itself was not successful, i.e. the QD-ligand mixture did not become transparent over time. In summary, the results clearly show that side-functionalized siloxane ligands are preferred.

The chemical compatibility will be discussed in more detail below:

Molecular n Reference Type Viscosity Weight (x1 or y1) AMINES AB250914 Polydimethylsiloxane, monoaminopropyl 7-14 cSt.  800-1100 14 terminated; 95%, viscosity AB111872 Polydimethylsiloxane, aminopropyl terminated; 20-30 cSt  900-1000 14 viscosity AB109369 Polydimethylsiloxane, aminopropyl terminated; 10-15 cSt. 850-900 12 3.2-3.8% amine, viscosity AB153374 Polydimethylsiloxane, aminopropyl terminated; 900-1100 cSt 15000-20000 236 0.11-0.12% amine, viscosity AB109373 (2-3% Aminopropylmethylsiloxane) - 80-200 cSt. 4000-6000 68 dimethylsiloxane copolymer; viscosity AB109374 (4-5% Aminopropylmethylsiloxane) - 150-300 cSt. 7000-9000 108 dimethylsiloxane copolymer; viscosity AB109371 Polydimethylsiloxane, aminopropyl terminated, 100-120 5000-7000 378 0.6-0.7% amine; AB129258 3-Aminopropylpentamethyldisiloxane; 97% 2 AB110832 1,3-Bis(3-aminopropyl)tetramethyldisiloxane; 2 97% AB129067 3-Aminopropylmethylbis(trimethylsiloxy)silane; 2 97% AB116669 (0.5-1.5% 100-200 cSt 4000-6000 68 AMINOETHYLAMINOPROPYLMETHOXYSILOXANE) - DIMETHYLSILOXANE COPOLYMER WITH BRANCH STRUCTURE, AB116668 (2-4% 900-1,200 cSt 28000 81 AMINOETHYLAMINOPROPYLMETHYLSILOXANE) - DIMETHYLSILOXANE COPOLYMER, AB124359 (2-4% 200-300 6000-9000 101 AMINOETHYLAMINOPROPYLMETHOXYSILOXANE) - DIMETHYLSILOXANE COPOLYMER WITH BRANCH STRUCTURE ACIDS AB252409 Monocarboxydecyl terminated 20 cSt. 1200-1800 17 polydimethylsiloxane, asymmetric, AB128267 Polydimethylsiloxane, carboxydecyldimethyl 15-50 cSt. 1000 17 terminated; viscosity AB116682 Polydimethylsiloxane, carboxypropyl terminated,  800-1200 28000 378 AB111537 Trimethylsilylpropionic acid; 97% ~1 142 1 AB110833 1,3-Bis(3-carboxypropyl)tetramethyldisiloxane; 2 97% AB111511 2-(Trimethylsilyl)acetic acid 2 Custom Synthesized Ligand with a Carboxylic Acid Group, Based on the AB109373 Molecule

The successful examples above of ligands with functional groups in the side-chain are based on amine functional groups. However, it is known that carboxylic acid side-groups (especially in fact the carboxylate groups) are more preferred in view of QD-stability. In addition, it is observed that excess amine ligands inhibit the silicone curing reaction, whereas excess carboxylic acid ligands don't do so. Thus, a molecule similar to e.g. AB109373 but with a carboxylic acid instead of an amine side-group and with a viscosity of 100-300 cSt and similar molecular weight is most preferred. The molecule may be branched or not branched. However, such molecules are not available commercially to the best of our knowledge. We have therefore custom-synthesized this molecule by reacting an anhydride moiety to the amine group of the AB109373 ligand. The reaction and resulting carboxy-ligand is shown in FIG. 3h (conversion of the AB109373 ligand into a carboxy-functionalized ligand by reaction with succinic anhydride).

We have tested the custom synthesized AB109373-COOH ligand. Upon mixing QDs with the AB109373-COOH ligand a poor miscibility was again observed. However, the mixture became transparent already after a few minutes at 100° C. This higher reactivity may be due to the carboxylic acid group compared to an amine group, or the fact that the side-chain is slightly more extended. This is an important proof that also PDMS ligands with a carboxylic side-chain group can be used. Before mixing into other silicones, the system of the wavelength converter nano particles with siloxane grafting ligands (in toluene) was purified by substantially removing excess ligand.

TEM on Dried-in QDs with the AB109373 Custom Ligand on a Grid—Evidence for Ligand Exchange Including Washing Procedure.

FIG. 4a shows a TEM image of washed CdSe quantum dots, prepared following the method as described by Lim et al. (Advanced Materials, 2007, 19, p. 1927-1932), and dried on a TEM grid. FIG. 4b shows a TEM image of CdSe quantum dots, from the same batch as shown in FIG. 4a , after ligand exchange with the AB109373 custom ligand. FIG. 4a shows that the quantum dots before ligand exchange have an interparticle distance of about 1 nm. FIG. 4b shows that the quantum dots after ligand exchange and formed in 2-dimensional aggregates during the drying process, are placed well apart from each other having an interparticle distance of about 8 nm. The latter indicates that the thickness of the ligand layer on the quantum dots is around 4 nm.

TEM of QDs without Ligand Exchange in Various Silicones—Evidence for Big Aggregates

If CdSe quantum dots, as prepared according to the method of Lim et. al. (Advanced Materials, 2007, 19, p. 1927-1932), are mixed into silicones often immediately strong flocculation is observed. Large aggregates are formed and even if layers appear transparent to the naked eye, microscopy reveals that aggregates are present.

Examples of QDs with Long Ligand (x1=68) in Siloxanes with Various Chain-Lengths, Using Purified QDs

The purified QDs redispersed in toluene could be mixed with pdms with low molecular weights to create stable dispersions. The dots could be dispersed in a pdms with a molecular weight of 1250, 2000, 3780 and 5970 (corresponding viscosities of 10, 20, 50 and 100 cSt or 17, 27, 50, 80 Si backbone units)) to yield non-scattering dispersions. In a 200 cSt (Mw 9430, y1=127) pdms the sample was scattering.

Examples of QDs with Short Ligand in Siloxanes with Various Chain-Lengths

We modified QDs with a short end-functionalized siloxane ligand (monoaminopropyl terminated, AB 250914 with a viscosity of 7-14 cSt and Mw of 800-1100, x1=12)). The ligand exchange was successful, i.e. the QD-ligand mixture was transparent after ligand exchange (>12 h @ 100° C.). It was observed that upon mixing, the QDs with the AB250914 ligand remain fully transparent in the 10 cSt PDMS (y1=17) for about 30 seconds, after which it starts to flocculate gradually, and becomes turbid eventually. This result was reproduced several times. It is interpreted as that the ligands are just below the threshold of being sufficiently long to stabilize the QDs in the 10 cSt PDMS. As a result, the mixture is initially stable, but slowly starts to flocculate over time. When mixed with 100 cSt, (y1=80) this gave a turbid suspension immediately, as expected.

Examples of QDs with Long Ligand in Siloxanes with Various Type of Siloxane Groups

To investigate the importance of chemical compatibility between the ligand and siloxane polymer, the QDs with the custom AB109373-COOH ligand (x1=68) were mixed with siloxane molecules with various side groups. An overview is given in the table below.

The results in the table below once more confirm that the QDs with the AB109373-COOH ligand form a transparent mixture with PDMS siloxane molecules up to a viscosity of 100 cSt (y1=80). However, when the same QDs with AB109373-COOH ligands are mixed with other siloxane molecules with various side chains. As we have learned in the previous examples, the QDs with AB109373-COOH ligand (see also the table above) will not give transparent mixtures with siloxane molecules with a viscosity much higher than 100 cSt. However, the table below shows that even for siloxanes with relatively low viscosity, these QDs with AB109373-COOH ligand give turbid mixtures. This indicates that the siloxane backbone of the ligand preferably matches chemically with the backbone of the siloxane polymer it is to be mixed with. In this case, our AB109373-COOH ligand has 100% dimethyl groups, so that it disperses well in the 100% PDMS siloxanes. It is foreseen that in case the QDs need to be mixed with e.g. a phenylmethyl (50%) silicone, the ligands on the QDs should have a similar e.g. phenylmethyl (50%) siloxane backbone.

+custom +custom ligand ligand + QDs Side-chain Visc. (cSt.) (no QDs) (LE) AB109355 dimethyl, vinyl term. 4-8 Clear Clear AB109356 dimethyl 100 Clear Clear AB109358 dimethyl 1000 Hazy Hazy AB123773 Octyl, 100%  600-1000 Turbid Turbid AB252388 Ethylmethyl (80%), 1200 Turbid Turbid 2-phenylpropylmethyl (20%) AB252389 Hexylmethyl (50%) 1500-2000 Phase Sep Turbid 2-phenylpropylmethyl (50%) AB252390 Dodecylmethyl (65%) 2- 1200 Clear/Phase Sep Phase Sep phenylpropylmethyl (35%) AB252387 2-phenylpropylmethyl 1000 Turbid Turbid (100%) AB128982 Phenylmethyl (100%) 100 Turbid Turbid AB109385 Phenylmethyl (50%) 125 Turbid Turbid AB181560 Phenylmethyl (60%)* 500 Turbid Turbid

Example of a Curable Composition Using Low Molecular Weight Silicones

To prepare cured layers to following composition was used:

Product Code Name, type amount AB109389 (Mw 25/35% methylhydrosiloxane 0.2 ml 2000-3000) AB109356 (about Pdms, vinyldimethoxysiloxy 0.8 ml 5000 Mw) terminated AB146697 Platinum divinylmethyldisiloxane 20 μl of a 1:300 in xylene dilution in xylene

Layers were prepared using more concentrated QDs (CdSe) silicone liquids using dots redispersed in 100 μl toluene, the dots (CdSe) having AB109373-COOH grafting ligands. The liquid was drop casted on eagle glass slides 3*3 cm, precleaned with iso-propanol and acetone. The layers were cured for one hour at 150° C. resulting in a transparent colored film. A bulk gel was prepared by curing for 2 hours at 70° C. Transparent layers and bulk materials of 5 mm thickness were obtained, as shown in FIG. 5a , where the transparency of the material is demonstrated by placing it on top of a piece of text where the word “meet” is clearly visible. In another experiment a bulk gel was prepared using identical CdSe QDs that were not modified, i.e. not having AB109373-COOH grafting ligands, and using an identical silicone liquid composition as indicated in the table directly above, wherein the resulting bulk gel looked very turbid and having relatively big aggregates of quantum dots.

Effect of Length of Short Chain Siloxane Polymers on Dispersability of Grafted Nanoparticle

Ligand exchanged (Mw=5000 g/mol/viscosity=100 cSt. siloxane ligand with a COOH functionalised side group) quantum dots were purified using 200 cSt pdms and subsequently washed in toluene/ethanol mixtures, to remove the excess of the ligand used for the exchange. After redispersion in toluene they were added to 10, 20, 50, 100 and 200 cSt pdms (quantities: 100 ul QDs in tol to 1 ml of PDMS i.e. 10% solvent). The dispersion remained transparent indicating colloidal stability, except for the 200 cSt pdms which gave a turbid suspension. The molecular weight of the 200 cSt pdms is about 9,500 which exceeds that of the ligand, while the other pdms types are of equal or lower molecular weight than that of the ligand.

Effect of Solvent (1)

To the Qdots in 50 cSt pdms with 10% solvent still present, 350 cSt PDMS was added in a 1:1 ratio. The dispersion remained clear indicating colloidal stability for at least two hours. The mixtures PDMS turned turbid overnight.

Effect of Solvent (2)

Toluene was removed from the Qdots in 50 cSt PDMS (see above) using vacuum. The quantum dots in the polymer melt remained clear. Mixing with 350 cSt and 1000 cSt pdms in a 1:1 ratio the dispersions remained clear indicating colloidal stability for at least two hours. After 20 hours the mixture with 1000 cSt pdms turned slightly turbid, but the mixture with the 350 cSt remained clear for more than 1 day. This indicates that removal of solvent helps stabilizing the QDs in these PDMS mixtures. In general it can be stated that the presence of solvent in the QD-PDMS melts is detrimental to colloidal stability. Mixing with KJR9226 (viscosity 1000 cSt.) resulted in a dispersion that was optically almost clear. The slight turbidity is also caused by the fact that KJR9226 has some turbidity by itself. Without removal of toluene, or when the ligand exchanged quantum dots were mixed in with KJR9226 directly, a much more strongly scattering dispersion was obtained, demonstrating the relevance of solvent removal. Without any ligand modification also highly scattering dispersions are obtained.

Example of Making Product (1)

Ligand exchanged quantum dots in toluene were dispersed in a 1:1 mixtures of vinyl and hydride terminated pdms with a viscosity around 8 cSt. Toluene was removed under vacuum and the mixtures was added to a commercially available silicone, KJR9226. A layer was dropcasted onto a glass substrate and thermally cured at 180° C. for 3 hours. A brightly coloured layer with low degree of scattering was obtained (see FIG. 5b ).

For the next Examples 2-5 applies: CdSe based Quantum dots were ligand exchanged with custom-ligand 373 and washed as described above.

Example of Making Product (2)

70 μl of a 0.5% quantum dot solution in toluene was added to 200 μl of a 50/50 mixture of pdms vinyldimethylsiloxy terminated with a viscosity of 4-8 cSt (M_(w)=770) and pdms, hydride terminated of 7-10 cSt (no M_(w) information available, from the viscosity data supplied it can be deduced that it will be at least as high as that of the pdms vinyldimethylsiloxy terminated with a viscosity of 4-8 cSt, consequently >770). The sample was placed under vacuum for one hour to remove toluene. A volume 200 μl of degassed KJR9226D were added to the clear solution. After mixing a layer was drop-casted on a glass substrate and a bulk gel was prepared in a glass tube. They were both cured at 150° C. for 30 minutes, resulting in a transparent layer and a transparent silicone gel. After 20 minutes the bulk gel was still partly liquid, but completely transparent to the eye.

Example of Making Product (3)

70 μl of a 0.5% quantum dot solution in toluene was added to 200 μl of a 80/20 mixture of pdms vinyldimethylsiloxy terminated with a viscosity of 100 cSt (M_(w)=6000) and 25/35% methylhydrosiloxane of 25-35 cSt (no M_(w) information available). The sample was placed under vacuum for one hour to remove toluene. A volume 200 μl of degassed KJR9226D were added to the clear solution. After mixing a layer was drop-casted on a glass substrate and a bulk gel was prepared in a glass tube. They were both cured at 150° C., resulting in a transparent layer and a transparent silicone gel. After 20 minutes the bulk gel was still partly liquid, but completely transparent to the eye.

Example of Making Product (4)

70 μl of a 0.5% quantum dot solution in toluene was added to 200 μl of a 80/20 mixture of pdms vinyldimethylsiloxy terminated with a viscosity of 100 cSt (M_(w)=6000) and pdms, hydride terminated of 7-10 cSt (no M_(w) information available, from the viscosity data supplied it can be deduced that it will be at least as high as that of the pdms vinyldimethylsiloxy terminated with a viscosity of 4-8 cSt, consequently >770). The sample was placed under vacuum for one hour to remove toluene. A volume 200 μl of degassed KJR9226D were added to the clear solution. After mixing a layer was drop-casted on a glass substrate and a bulk gel was prepared in a glass tube. They were both cured at 150° C., resulting in a transparent layer and a transparent silicone gel. (When checked after 20 minutes the gel was already solidified).

Example of Making Product (5)

70 μl of a 0.5% quantum dot solution in toluene was added to 200 μl of a 80/20 mixture of pdms vinyldimethylsiloxy terminated with a viscosity of 4-8 cSt (M_(w)=770) and 25/35% methylhydrosiloxane of 25-35 cSt (no M_(w) information available). The sample was placed under vacuum for one hour to remove toluene. A volume 200 μl of degassed KJR9226D were added to the clear solution. After mixing a layer was drop-casted on a glass substrate and a bulk gel was prepared in a glass tube. They were both cured at 150° C., resulting in a transparent layer and a transparent silicone gel. (When checked after 20 minutes the gel was already solidified).

The above examples demonstrate that transparent layers and gels can be obtained by using cross-linkable silicones of a lower or equal molecular weight as the ligand on the quantum dot surface and this allows subsequent mixing with a commercially available silicone such as KJR9226 to form transparent layers and gels.

TABLE I Curable pdms compounds (according to the Examples given above); Product Code Name, type Viscosity (cSt) AB109355 pdms, vinyldimethylsiloxy term 4-8 cSt AB146376 Pdms, hydride terminated 7-10 cSt AB109389 25/35% methylhydrosiloxane 25-35 cSt AB109356 Pdms, vinyldimethylsiloxy terminated 100 cSt 

The invention claimed is:
 1. A process for the production of a wavelength converter comprising a siloxane polymer matrix with wavelength converter nanoparticles embedded therein, the process comprising: grafting siloxane grafting ligands to wavelength converter nanoparticles to form grafted wavelength converter nanoparticles; mixing short chain siloxane polymers with the grafted wavelength converter nanoparticles to form a first liquid having grafted wavelength converter nanoparticles dispersed in the short chain siloxane polymers, wherein the short chain siloxane polymers are non-grafting ligands; mixing the first liquid with curable siloxane polymers, wherein the curable siloxane polymers are non-grafting ligands; and curing the curable siloxane polymers, thereby producing the wavelength converter comprising the short chain siloxane polymers and the curable siloxane polymers that do not graft onto the grafted wavelength converter nanoparticles; wherein the short chain siloxane polymers have s1 Si backbone elements, wherein the siloxane grafting ligands comprise siloxane grafting ligands having x1 Si backbone elements, wherein at least one non-terminal Si backbone element of each siloxane grafting ligand comprises a side group having a grafting functionality; and wherein the curable siloxane polymers have y1 Si backbone elements; wherein x1/s1≥0.8, s1<y1 and wherein x1<y1.
 2. The process according to claim 1, wherein s1<x1<y1, wherein s1 is at least 7, wherein x1 is at least 20, and wherein y1 is at least
 100. 3. The process according to claim 1, wherein the first liquid comprises one or more solvents in a total amount of not more than 2 wt. %.
 4. The process according to claim 1, wherein the process further includes removing a solvent from the first liquid, before mixing the first liquid and the curable siloxane polymers.
 5. The process according to claim 1, wherein the curable siloxane polymers comprise unsaturated bond comprising groups and hydride groups in a first unsaturated bond hydride group molar ratio, and wherein the short chain siloxane polymer comprise unsaturated bond comprising groups and hydride groups in a second unsaturated bond hydride group molar ratio, wherein the second unsaturated bond hydride group molar ratio is in the range of 50% to 150% of the first unsaturated bond hydride group molar ratio.
 6. The process according to claim 1, wherein at least 70% of the Si backbone elements of the short chain siloxane polymers have methyl side groups, wherein at least 90% of the Si backbone elements of the siloxane grafting ligands have methyl side groups and wherein at least 90% of the Si backbone elements of the siloxane polymers have methyl side groups.
 7. The process according to claim 1, wherein the short chain siloxane polymers, the siloxane grafting ligands and the curable siloxane polymers are poly dimethyl siloxanes, or poly diphenyl siloxanes, or poly methylphenyl siloxanes.
 8. The process according to claim 1, wherein the side group having a grafting functionality is selected from the group consisting of an amine comprising side group, a carboxylate comprising side group, a phosphine comprising side group, a phosphine oxide comprising side group, a phosphate comprising side group, and a thiol comprising side group.
 9. The process according to claim 1, wherein the wavelength converter nanoparticles are selected from the group consisting of core-shell nano particles, with the cores and shells comprising one or more of CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, HgS, HgSe, HgTe, CdSeS, CdSeTe, CdSTe, ZnSeS, ZnSeTe, ZnSTe, HgSeS, HgSeTe, HgSTe, CdZnS, CdZnSe, CdZnTe, CdHgS, CdHgSe, CdHgTe, CdHgZnS, HgZnSe, HgZnTe, CdZnSeS, CdZnSeTe, CdZnSTe, CdHgSeS, CdHgSeTe, CdHgSTe, HgZnSeS, HgZnSeTe, HgZnSTe, GaN, GaP, GaAs, AlN, AlP, AlAs, InN, InP, InAs, GaNP, GaNAs, GaPAs, AlNP, AlNAs, AlPAs, InNP, InNAs, InPAs, GaAlNP, GaAlNAs, GaAlPAs, GaInNP, GaInNAs, GaInPAs, InAlNP, InAlNAs, and InAlPAs.
 10. A wavelength converter comprising a siloxane polymer matrix with wavelength converter nanoparticles embedded therein, wherein: the wavelength converter nanoparticles have an outer surface grafted with siloxane grafting ligands, and the siloxane polymer matrix comprises siloxane polymers of a first type and siloxane polymers of a second type, wherein at least part of the second type of siloxane polymers are cross-linked, wherein the siloxane polymers of the first type and the second type are non-grafting ligands that do not graft onto the grafted wavelength converter nanoparticles; wherein the siloxane polymers of the first type comprise short chain siloxane polymers having s1 Si backbone elements, wherein the siloxane grafting ligands comprise siloxane grafting ligands having x1 Si backbone elements, wherein at least one non-terminal Si backbone element of each siloxane grafting ligand comprises a side group having a grafting functionality, wherein the siloxane polymers of the second type comprise siloxane polymers having y1 Si backbone elements; wherein x1/s1≥0.8, s1<y1 and wherein x1<y1.
 11. The wavelength converter according to claim 10, wherein the side group having a grafting functionality is selected from the group consisting of an amine comprising side group or a carboxylate comprising side group, wherein s1<x1<y1, wherein s1 is at least 7, wherein x1 is at least 20, and wherein y1 is at least
 100. 12. The wavelength converter according to claim 10, wherein at least part of the second type of siloxane polymers are cross-linked, wherein also at least part of the first type of siloxane polymers are cross-linked, and wherein at least part of the first type of siloxane polymers and the second type of siloxane polymers are cross-linked with each other.
 13. The wavelength converter according to claim 10, wherein at least 70% of the Si backbone elements of the short chain siloxane polymers have methyl side groups, wherein at least 90% of the Si backbone elements of the siloxane grafting ligands have methyl side groups, wherein at least 90% of the Si backbone elements of the siloxane polymers have methyl side groups, wherein the wavelength converter nanoparticles are selected from the group consisting of core-shell nano particles, with the cores and shells comprising one or more of CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, HgS, HgSe, HgTe, CdSeS, CdSeTe, CdSTe, ZnSeS, ZnSeTe, ZnSTe, HgSeS, HgSeTe, HgSTe, CdZnS, CdZnSe, CdZnTe, CdHgS, CdHgSe, CdHgTe, HgZnS, HgZnSe, HgZnTe, CdZnSeS, CdZnSeTe, CdZnSTe, CdHgSeS, CdHgSeTe, CdHgSTe, HgZnSeS, HgZnSeTe, HgZnSTe, GaN, GaP, GaAs, AlN, AlP, AlAs, InN, InP, InAs, GaNP, GaNAs, GaPAs, AlNP, AlNAs, AlPAs, InNP, InNAs, InPAs, GaAlNP, GaAlNAs, GaAlPAs, GaInNP, GaInNAs, GaInPAs, InAlNP, InAlNAs, and InAlPAs, and wherein the wavelength converter comprises one or more remaining solvents, with a total amount of the solvents of not more than 1 wt. %.
 14. A lighting device comprising: a light source configured to generate light source light, a wavelength converter obtainable by the method of claim 1 configured to convert at least part of the light source light into visible converter light.
 15. A liquid crystal display device comprising a back lighting unit, wherein the back lighting unit comprises one or more lighting devices according to claim
 14. 16. The method according to claim 1, wherein the at least one non-terminal Si backbone element of each siloxane grafting ligand comprising a side group having a grafting functionality is positioned between 20% and 80% of a length of the Si backbone.
 17. The method according to claim 1, further comprising: purifying the grafted wavelength converter nanoparticles to remove ungrafted siloxane grafting ligands before mixing the short chain siloxane polymers with the grafted wavelength converter nanoparticles.
 18. The method according to claim 1, wherein only non-terminal backbone Si elements of the siloxane grating ligand comprise side groups having grafting functionality.
 19. The method according to claim 10, wherein only non-terminal backbone Si elements of the siloxane grating ligand comprise side groups having grafting functionality. 